kunit: kunit_parser: make parser more robust

Previously, kunit_parser did not properly handle kunit TAP output that
- had any prefixes (generated from different configs e.g.
CONFIG_PRINTK_TIME)
- had unrelated kernel output mixed in the middle of
it, which has shown up when testing with allyesconfig
To remove prefixes, the parser looks for the first line that includes
TAP output, "TAP version 14". It then determines the length of the
string before this sequence, and strips that number of characters off
the beginning of the following lines until the last KUnit output line is
reached.
These fixes have been tested with additional tests in the
KUnitParseTest and their associated logs have also been added.

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