kunit: tool: Fix bug in parsing test plan

kunit: tool: Fix bug in parsing test plan

A bug was identified where the KTAP below caused an infinite loop:

 TAP version 13
 ok 4 test_case
 1..4

The infinite loop was caused by the parser not parsing a test plan
if following a test result line.

Fix this bug by parsing test plan line to avoid the infinite loop.

Link: https://lore.kernel.org/r/20250313192714.1380005-1-rmoar@google.com
Signed-off-by: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <shuah@kernel.org>