iotests: Write test output to TEST_DIR

iotests: Write test output to TEST_DIR

Drop the use of OUTPUT_DIR (test/qemu-iotests under the build
directory), and instead write test output files (.out.bad, .notrun, and
.casenotrun) to TEST_DIR.

With this, the same test can be run concurrently without the separate
instances interfering, because they will need separate TEST_DIRs anyway.
Running the same test separately is useful when running the iotests with
various format/protocol combinations in parallel, or when you just want
to aggressively exercise a single test (e.g. when it fails only
sporadically).

Putting this output into TEST_DIR means that it will stick around for
inspection after the test run is done (though running the same test in
the same TEST_DIR will overwrite it, just as it used to be); but given
that TEST_DIR is a scratch directory, it should be clear that users can
delete all of its content at any point.  (And if TEST_DIR is on tmpfs,
it will just disappear on shutdown.)  Contrarily, alternative approaches
that would put these output files into OUTPUT_DIR with some prefix to
differentiate between separate test runs might easily lead to cluttering
OUTPUT_DIR.

(This change means OUTPUT_DIR is no longer written to by the iotests, so
we can drop its usage altogether.)

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220221172909.762858-1-hreitz@redhat.com>
[hreitz: Simplified `Path(os.path.join(x, y))` to `Path(x, y)`, as
         suggested by Vladimir; and rebased on 9086c7639822b6
         ("tests/qemu-iotests: Rework the checks and spots using GNU
         sed")]
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>