As `-fno-show-warning-groups` shows associated warning groups regardless
of whether the respective warning group flag as been passed on the CLI,
the warning-group information may be confusing to users.

At this point, `-fshow-warning-groups` is useful mostly to GHC
developers and possibly GHC users who want to see which warning groups
an emitted warning is part of. (Btw, this is particularly interesting in
combination with `-Weverything` which enables *every* warning flag known
to GHC.)

Consequently, starting with this commit, one has to opt-in via
`-fshow-warning-groups` for GHC to show warning groups.

In order to reduce the testsuite delta in this commit, the
`-fshow-warning-groups` flag has been added to TEST_HC_OPTS.