oeqa/core/runner: add helper to know about expected failures

oeqa/core/runner: add helper to know about expected failures

Testing framework currently uses the unittest.expectedFailure decorator for
tests that can have intermittent failures (see PTEST_EXPECT_FAILURE = "1")
in core-image-ptest.bb. While it allows upper layers to run tests without
failing on "fragile" tests, it prevents those from knowing more about those
failing tests since they are not accounting as failures (for example we
could want to retrieve some logs about failed tests to improve them, and
eventually to drop expectFailure decorator)

Add a helper to allow upper layers to know about those failures which won't
make global testing session

Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>