iotests: Let _make_test_img guess $TEST_IMG_FILE

iotests: Let _make_test_img guess $TEST_IMG_FILE

When most iotests want to create a test image that is named differently
from the default $TEST_IMG, they do something like this:

    TEST_IMG="$TEST_IMG.base" _make_test_img $options

This works fine with the "file" protocol, but not so much for anything
else: _make_test_img tries to create an image under $TEST_IMG_FILE
first, and only under $TEST_IMG if the former is not set; and on
everything but "file", $TEST_IMG_FILE is set.

There are two ways we can fix this: First, we could make all tests
adjust not only TEST_IMG, but also TEST_IMG_FILE if that is present
(e.g. with something like _set_test_img_suffix $suffix that would affect
not only TEST_IMG but also TEST_IMG_FILE, if necessary).  This is a
pretty clean solution, and this is maybe what we should have done from
the start.

But it would also require changes to most existing bash tests.  So the
alternative is this: Let _make_test_img see whether $TEST_IMG_FILE still
points to the original value.  If so, it is possible that the caller has
adjusted $TEST_IMG but not $TEST_IMG_FILE.  In such a case, we can (for
most protocols) derive the corresponding $TEST_IMG_FILE value from
$TEST_IMG value and thus work around what technically is the caller
misbehaving.

This second solution is less clean, but it is robust against people
keeping their old habit of adjusting TEST_IMG only, and requires much
less changes.  So this patch implements it.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20201027190600.192171-15-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>