run_unittests: check for pytest and pytest-xdist separately
run_unittests: check for pytest and pytest-xdist separately

Do not quit from using pytest at all, when pytest is present, simply
because xdist isn't available. Even without xdist, pytest is still
useful.

There doesn't seem to be any particular reason to require xdist. It just
happens to have been implemented that way, back in commit
4200afc74d1e6ba6d117e900799d0d82a85bae8a when we originally added a
check to avoid pytest erroring out with unknown options when xdist
options are passed and xdist is not installed.

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