Currently, `run_kernel()` dumps all the kernel output to a file
(.kunit/test.log) and then opens the file and yields it to callers.
This made it easier to respect the requested timeout, if any.

But it means that we can't yield the results in real time, either to the
parser or to stdout (if --raw_output is set).

This change spins up a background thread to enforce the timeout, which
allows us to yield the kernel output in real time, while also copying it
to the .kunit/test.log file.
It's also careful to ensure that the .kunit/test.log file is complete,
even in the kunit_parser throws an exception/otherwise doesn't consume
every line, see the new `finally` block and unit test.

For example:

$ ./tools/testing/kunit/kunit.py run --arch=x86_64 --raw_output
<configure + build steps>
...
<can now see output from QEMU in real time>

This does not currently have a visible effect when --raw_output is not
passed, as kunit_parser.py currently only outputs everything at the end.
But that could change, and this patch is a necessary step towards
showing parsed test results in real time.
Signed-off-by: NDaniel Latypov <dlatypov@google.com>
Reviewed-by: NDavid Gow <davidgow@google.com>
Reviewed-by: NBrendan Higgins <brendanhiggins@google.com>
Signed-off-by: NShuah Khan <skhan@linuxfoundation.org>