src.conf: Add a MK_ZFS_TESTS knob

[markj]

Summary

The in-tree ZFS test suite is somewhat outdated and I see a number of
failures there. I tend to think that we want to integrate the OpenZFS
test suite somehow, replacing the legacy one, though it's also possible
to run that as a separate test suite.

In any case, if one wants to run the OpenZFS test suite separately, it's
useful to be able to disable installation of the legacy ZFS test suite,
so let's provide a src.conf option to do that.

[emaste]

tools/build/options/WITHOUT_ZFS

9–10

This text should be generated automatically by makeman

[markj]

Remove an unneeded doc update.

[markj]

it's useful to be able to disable installation of the legacy ZFS test suite

One other reason it's useful is to save time - the ZFS tests take a very long time to run when the kernel is compiled with sanitizers enabled.

[markj]

Any objections to this? I'd like to have a separate discussion about the future of the "legacy" test suite versus OpenZFS' test suite, but this option is useful to me regardless.

[asomers]

This seems reasonable. Jenkins already disables the ZFS tests for normal runs.