tests/selftest: Add a new pseudo flaky test.

This test is to simulate the behavior of a flaky test. This will be required
when we will make some modifications to the pass/fail metric calculation of
the test infrastructure where we will need a test with non-zero pass
and non-zero failure rate.

Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Nirjhar Roy (IBM) <nirjhar.roy.lists@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>