Documentation: kunit: Add some troubleshooting tips to the FAQ

Documentation: kunit: Add some troubleshooting tips to the FAQ

Add an FAQ entry to the KUnit documentation with some tips for
troubleshooting KUnit and kunit_tool.

These suggestions largely came from an email thread:
https://lore.kernel.org/linux-kselftest/41db8bbd-3ba0-8bde-7352-083bf4b947ff@intel.com/T/#m23213d4e156db6d59b0b460a9014950f5ff6eb03

Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>