compare_dg_tests.pl: Add --flaky-tests= option

This is similar to --unstable-tests, but reads a file produced by
validate_failures.py.

Such tests are ignored in the comparison results, but displayed when
using the long format (-l).

We currently ignore the expire=YYYYMMDD attribute.

Change-Id: I5bcdd2df9cc19a880f6196c3ca8fd62dce2961f8
7 files changed