kunit: test: Improve error messages for kunit_tool when kunitconfig is invalid

Previous error message for invalid kunitconfig was vague. Added to it so
that it lists invalid fields and prompts for them to be removed.  Added
validate_config function returning whether or not this kconfig is valid.

Signed-off-by: Heidi Fahim <heidifahim@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>