iotests: Add -o and --no-opts to _make_test_img

iotests: Add -o and --no-opts to _make_test_img

Blindly overriding IMGOPTS is suboptimal as this discards user-specified
options.  Whatever options the test needs should simply be appended.

Some tests do this (with IMGOPTS=$(_optstr_add "$IMGOPTS" "...")), but
that is cumbersome.  It’s simpler to just give _make_test_img an -o
parameter with which tests can add options.

Some tests actually must override the user-specified options, though,
for example when creating an image in a different format than the test
$IMGFMT.  For such cases, --no-opts allows clearing the current option
list.

Signed-off-by: Max Reitz <[email protected]>
Reviewed-by: Maxim Levitsky <[email protected]>
Message-id: 20191107163708[email protected]
Signed-off-by: Max Reitz <[email protected]>