Add test for racy git detection in FileSnapshot

Add test for racy git detection in FileSnapshot

Repeat the test 10000 times to get statistics if measured
fsTimestampResolution is working in practice to detect racy git
situations.

Add a class to compute statistics for this test. Log delta between
lastModified and time when FileSnapshot failed to detect modification.
This happens if the racy git limit determined by measuring filesystem
timestamp resolution and clock resolution is too small. If it would be
correct FileSnapshot would always detect modification or mark it
modified if time since modification is smaller than the racy git limit.

Change-Id: Iabe7af1a7211ca58480f8902d4fa4e366932fc77
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>