Auto merge of #19440 - asajeffrey:test-perf-utf8, r=jdm
Auto merge of #19440 - asajeffrey:test-perf-utf8, r=jdm

Added explicit UTF-8 encoding to test-perf.

<!-- Please describe your changes on the following line: -->

Added explicit UTF-8 encoding to test-perf, to try to avoid:
```
Traceback (most recent call last):
  File "runner.py", line 371, in <module>
    main()
  File "runner.py", line 361, in main
    save_result_csv(results, args.output_file, testcases, args.runs, args.base)
  File "runner.py", line 287, in save_result_csv
    writer.writerows(results)
  File "/usr/lib/python3.4/csv.py", line 159, in writerows
    return self.writer.writerows(rows)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 101-107: ordinal not in range(128)
```
http://build.servo.org/builders/linux-nightly/builds/526/steps/test/logs/stdio

---
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- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because this is test infrastructure

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peterjoel/servo,CJ8664/servo,saneyuki/servo,mbrubeck/servo,mbrubeck/servo,thiagopnts/servo,ConnorGBrewster/servo,szeged/servo,anthgur/servo,rnestler/servo,pyfisch/servo,ConnorGBrewster/servo,pyfisch/servo,larsbergstrom/servo,splav/servo,saneyuki/servo,CJ8664/servo,peterjoel/servo,upsuper/servo,notriddle/servo,rnestler/servo,DominoTree/servo,saneyuki/servo,larsbergstrom/servo,splav/servo,paulrouget/servo,paulrouget/servo,sadmansk/servo,peterjoel/servo,canaltinova/servo,jimberlage/servo,emilio/servo,canaltinova/servo,pyfisch/servo,KiChjang/servo,jimberlage/servo,nnethercote/servo,KiChjang/servo,CJ8664/servo,jimberlage/servo,ConnorGBrewster/servo,DominoTree/servo,emilio/servo,paulrouget/servo,notriddle/servo,thiagopnts/servo,KiChjang/servo,mattnenterprise/servo,mbrubeck/servo,jimberlage/servo,mattnenterprise/servo,cbrewster/servo,avadacatavra/servo,thiagopnts/servo,nnethercote/servo,peterjoel/servo,mattnenterprise/servo,rnestler/servo,anthgur/servo,KiChjang/servo,dati91/servo,SimonSapin/servo,saneyuki/servo,larsbergstrom/servo,canaltinova/servo,rnestler/servo,emilio/servo,jimberlage/servo,sadmansk/servo,upsuper/servo,mattnenterprise/servo,paulrouget/servo,KiChjang/servo,nnethercote/servo,CJ8664/servo,saneyuki/servo,DominoTree/servo,cbrewster/servo,notriddle/servo,notriddle/servo,saneyuki/servo,szeged/servo,sadmansk/servo,canaltinova/servo,CJ8664/servo,paulrouget/servo,larsbergstrom/servo,rnestler/servo,thiagopnts/servo,splav/servo,emilio/servo,fiji-flo/servo,fiji-flo/servo,anthgur/servo,avadacatavra/servo,pyfisch/servo,cbrewster/servo,nnethercote/servo,ConnorGBrewster/servo,mattnenterprise/servo,DominoTree/servo,emilio/servo,avadacatavra/servo,dati91/servo,szeged/servo,upsuper/servo,jimberlage/servo,KiChjang/servo,paulrouget/servo,avadacatavra/servo,avadacatavra/servo,notriddle/servo,CJ8664/servo,jimberlage/servo,mattnenterprise/servo,dati91/servo,anthgur/servo,saneyuki/servo,szeged/servo,SimonSapin/servo,mattnenterprise/servo,fiji-flo/servo,KiChjang/servo,rnestler/servo,splav/servo,ConnorGBrewster/servo,paulrouget/servo,dati91/servo,sadmansk/servo,szeged/servo,thiagopnts/servo,rnestler/servo,mbrubeck/servo,splav/servo,SimonSapin/servo,rnestler/servo,canaltinova/servo,larsbergstrom/servo,avadacatavra/servo,larsbergstrom/servo,szeged/servo,saneyuki/servo,peterjoel/servo,saneyuki/servo,fiji-flo/servo,paulrouget/servo,dati91/servo,szeged/servo,notriddle/servo,nnethercote/servo,upsuper/servo,nnethercote/servo,jimberlage/servo,SimonSapin/servo,splav/servo,SimonSapin/servo,mbrubeck/servo,fiji-flo/servo,larsbergstrom/servo,peterjoel/servo,larsbergstrom/servo,CJ8664/servo,szeged/servo,avadacatavra/servo,pyfisch/servo,anthgur/servo,emilio/servo,upsuper/servo,dati91/servo,DominoTree/servo,mbrubeck/servo,pyfisch/servo,sadmansk/servo,thiagopnts/servo,fiji-flo/servo,emilio/servo,canaltinova/servo,emilio/servo,upsuper/servo,emilio/servo,sadmansk/servo,DominoTree/servo,pyfisch/servo,cbrewster/servo,anthgur/servo,jimberlage/servo,ConnorGBrewster/servo,splav/servo,mbrubeck/servo,notriddle/servo,splav/servo,avadacatavra/servo,notriddle/servo,fiji-flo/servo,notriddle/servo,mbrubeck/servo,KiChjang/servo,nnethercote/servo,saneyuki/servo,upsuper/servo,upsuper/servo,KiChjang/servo,canaltinova/servo,pyfisch/servo,KiChjang/servo,nnethercote/servo,thiagopnts/servo,sadmansk/servo,szeged/servo,canaltinova/servo,cbrewster/servo,nnethercote/servo,larsbergstrom/servo,dati91/servo,peterjoel/servo,mattnenterprise/servo,fiji-flo/servo,thiagopnts/servo,splav/servo,sadmansk/servo,ConnorGBrewster/servo,nnethercote/servo,anthgur/servo,DominoTree/servo,larsbergstrom/servo,pyfisch/servo,peterjoel/servo,DominoTree/servo,cbrewster/servo,SimonSapin/servo,SimonSapin/servo,emilio/servo,peterjoel/servo,peterjoel/servo,paulrouget/servo,splav/servo,pyfisch/servo,ConnorGBrewster/servo,DominoTree/servo,SimonSapin/servo,dati91/servo,cbrewster/servo,szeged/servo,anthgur/servo,cbrewster/servo,paulrouget/servo,CJ8664/servo,DominoTree/servo,notriddle/servo