iotests: Replace IMGOPTS= by -o

Tests should not overwrite all user-supplied image options, but only add
to it (which will effectively overwrite conflicting values).  Accomplish
this by passing options to _make_test_img via -o instead of $IMGOPTS.

For some tests, there is no functional change because they already only
appended options to IMGOPTS.  For these, this patch is just a
simplification.

For others, this is a change, so they now heed user-specified $IMGOPTS.
Some of those tests do not work with all image options, though, so we
need to disable them accordingly.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-id: 20191107163708.833192-12-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>