kunit: tool: fix _list_tests filtering wrong variable when list has TAP prefix

`_list_tests()` runs the kernel to list tests, strips printk timestamp
lines via `extract_tap_lines()`, then drops the dummy TAP header from
the cleaned `lines`.  However the subsequent regex filter mistakenly
operates on the original `output` instead of the cleaned `lines`.  When
the kernel output includes timestamp prefixes (common with UML or slower
setups), e.g.:

  [    0.100000] suite.test1
  [    0.100000] suite.test2

the anchored regex `^[^\s.]+\.[^\s.]+$` rejects them and `--list_tests`
returns an empty list.

Filter `lines` instead of `output`, matching the behavior of the
adjacent `_list_tests_attr()` which already returns the cleaned list.

Add a regression test with timestamp-prefixed input to verify the fix.

Link: https://lore.kernel.org/r/20260803190059.36491-1-mohammad.abukhader@hotmail.com
Fixes: 723c8258c8fe ("kunit: tool: Add command line interface to filter and report attributes")
Signed-off-by: Mohammad Abu-Khader <mohammad.abukhader@hotmail.com>
Reviewed-by: David Gow <david@davidgow.net>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>