ci: Add support for running multiple tests simultaneously

To try this out, start a run like using multiple `--test`
arguments.

ci/run_test_suite.sh \
	--deqp \
	--host-gles \
	--backend vtest-gpu \
	--backend gpu \
	--test TEST1 \
        --test TEST2

Signed-off-by: Robert Foss <robert.foss@collabora.com>
7 jobs for multiple_tests in 15 minutes and 39 seconds (queued for 1 second)
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