ktest: Add IGNORE_WARNINGS to ignore warnings in some patches

Doing a patchcheck test, there may be warnings that gcc produces which
may be OK, and the test should not fail on that commit. By adding a
IGNORE_WARNINGS option to list a space delimited SHA1s that are ignored
lets the user avoid having the test fail on certain commits.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>