improve test for timestamp addition/subtraction (#27192)
improve test for timestamp addition/subtraction (#27192)

* improve test for timestamp addition/subtraction

* fix bug in test

* add test cases

pandas-dev/pandas,gfyoung/pandas,toobaz/pandas,TomAugspurger/pandas,TomAugspurger/pandas,datapythonista/pandas,toobaz/pandas,dsm054/pandas,TomAugspurger/pandas,jreback/pandas,dsm054/pandas,rs2/pandas,pandas-dev/pandas,jreback/pandas,jorisvandenbossche/pandas,rs2/pandas,jorisvandenbossche/pandas,gfyoung/pandas,jorisvandenbossche/pandas,dsm054/pandas,datapythonista/pandas,gfyoung/pandas,dsm054/pandas,jreback/pandas,jreback/pandas,jreback/pandas,gfyoung/pandas,gfyoung/pandas,rs2/pandas,toobaz/pandas,TomAugspurger/pandas,rs2/pandas,datapythonista/pandas,pandas-dev/pandas,toobaz/pandas,pandas-dev/pandas,datapythonista/pandas,jorisvandenbossche/pandas,dsm054/pandas,toobaz/pandas