depmod test fails when zstd not available

Hello,

kmod version 28 adds support for zstd. The support is optional but when
not enabled the depmod test which unconditionally tests zstd fails.

As kmod supports more and more compression methods I think it is not
reasonable to expect everyone has all copression libraries available.

Do you think it is reasonable to fix the tests to only use teh
compresion methods enabled by configure?

Thanks

Michal