See also: commit 0e6457f1bfe2 ("lib: Don't dump log buffer when
dynamic subtest failure is inherited")

This is quite an explicit top-to-bottom test that we don't get an
incorrect warn result for an innocent dynamic subtest. It is tested
here in runner_test instead of testing in lib/tests that the extra
lines are not printed, because not printing the extra lines is an
implementation detail that might change later. The issue is after all
about test results parsed by igt_runner.

v2: Squash adding the new mockup test binary to this commit

Signed-off-by: Petri Latvala <petri.latvala@intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>