iotest: Fix filtering order in 226

iotest: Fix filtering order in 226

The test directory should be filtered before the image format, otherwise
the test will fail if the image format is part of the test directory,
like so:

[...]
-can't open: Could not open 'TEST_DIR/t.IMGFMT': Is a directory
+can't open: Could not open '/tmp/test-IMGFMT/t.IMGFMT': Is a directory
[...]

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>