ktest: Add make_warnings_file and process full warnings

Although the patchcheck test checks for warnings in the files that were
changed, this check does not catch warnings that were caused by header
file changes and the warnings appear in C files not touched by the
commit.

Add a new option called WARNINGS_FILE. If this option is set, then the
file it points to is read before bulid, and the file should contain a
list of known warnings. If a warning appears in the build, this file is
checked, and if the warning does not exist in this file, then it fails
the build showing the new warning.

If the WARNINGS_FILE points to a file that does not exist, this will
cause any warning in the build to fail.

A new test is also added called "make_warnings_file". This test will
create do a build and record any warnings it finds into the
WARNINGS_FILE. This test is something that can be run before other tests
to build a warnings file of "known warnings", ie, warnings that were
there before your changes.

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