nixos/tests/opensmtpd: bump test (&build) timeout to 30m from 30s

I am not sure how this ever passed on hydra but 30s is barely enough to
pass the configure phase of opensmtpd. It is likely the package was
built as part of another jobset. Whenever it is built as part of the
test execution the timeout propagates and 30s is clearly not enough for
that.