As the feature name table can be quite large (over 9k if all 64 bits
of all three feature fields have names; a mere 8 features leaves only
8 bytes for a backing file name in a 512-byte cluster), it is unwise
to emit this optional header in images with small cluster sizes.
Update iotest 036 to skip running on small cluster sizes; meanwhile,
note that iotest 061 never passed on alternative cluster sizes
(however, I limited this patch to tests with output affected by adding
feature names, rather than auditing for other tests that are not
robust to alternative cluster sizes).
Signed-off-by: Eric Blake<address@hidden>