Add JUnit-style output format to run_unit_tests.py.
Add JUnit-style output format to run_unit_tests.py.

This patch adds a new output format to the test driver execution script,
which captures test case output, collects results and emits them in a
form that other tools can consume. This mode is enabled by specifying
"--junit=<file.xml>" to the script.

dbremner/bde,saxena84/bde,apaprocki/bde,osubboo/bde,RMGiroux/bde-allocator-benchmarks,che2/bde,jmptrader/bde,abeels/bde,che2/bde,minhlongdo/bde,che2/bde,bowlofstew/bde,apaprocki/bde,bloomberg/bde,idispatch/bde,frutiger/bde,bowlofstew/bde,abeels/bde,saxena84/bde,dharesign/bde,frutiger/bde,mversche/bde,bloomberg/bde-allocator-benchmarks,dbremner/bde,dbremner/bde,jmptrader/bde,saxena84/bde,jmptrader/bde,dharesign/bde,RMGiroux/bde-allocator-benchmarks,mversche/bde,frutiger/bde,bloomberg/bde-allocator-benchmarks,bowlofstew/bde,abeels/bde,apaprocki/bde,bloomberg/bde,RMGiroux/bde-allocator-benchmarks,minhlongdo/bde,gbleaney/Allocator-Benchmarks,apaprocki/bde,minhlongdo/bde,dbremner/bde,bloomberg/bde-allocator-benchmarks,apaprocki/bde,bowlofstew/bde,osubboo/bde,gbleaney/Allocator-Benchmarks,idispatch/bde,mversche/bde,gbleaney/Allocator-Benchmarks,minhlongdo/bde,frutiger/bde,abeels/bde,osubboo/bde,bloomberg/bde,bloomberg/bde-allocator-benchmarks,RMGiroux/bde-allocator-benchmarks,bloomberg/bde,abeels/bde,idispatch/bde,jmptrader/bde,abeels/bde,saxena84/bde,osubboo/bde,bloomberg/bde-allocator-benchmarks,RMGiroux/bde-allocator-benchmarks,idispatch/bde,dharesign/bde,che2/bde,gbleaney/Allocator-Benchmarks,mversche/bde,bloomberg/bde,dharesign/bde