First (?) performance regression unit test against clearing of a sheet.
First (?) performance regression unit test against clearing of a sheet.

I screwed this up earlier which made this operation take almost 50 seconds
to complete. It should finish in 0.00001 seconds or less.  The test checks
against 1 second, which should be enough of a buffer for slower machines.

Change-Id: I9923033045111c75a0740b6bb30a518fe93e01d2

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core