lib/kunit: Load test modules in background

For igt_runner to be able to correlate a stream of IGT results from
dynamic sub-subtests that correspond to individual kunit test cases, read
by the igt_runner from stdout / stderr of the test process, with a stream
of kernel messages read from /dev/kmsg, we need both result streams being
fed with data in parallel.  While our KTAP parser is currently started in
the background and reads KTAP results from /dev/kmsg in parallel with
execution of kunit tests performed by the kernel while we are loading a
kunit test module, results from the parser are then only stored as
intermediate data and not processed any further until the module loading
completes.  As a consequence, there is no synchronization between the two
streams.

Call the function that loads the kunit test module from a separate thread
and process the intermediate results immediately, as soon as available
from the background parser, without waiting for completion of module
loading.

Signed-off-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Acked-by: Mauro Carvalho Chehab <mchehab@kernel.org>