tests: avoid spurious failure for each gnu-fortran-using test

tests: avoid spurious failure for each gnu-fortran-using test

The tests compare pre- and post-run lists of envvars, which must
be the same, modulo a list of known, filtered-out exceptions.
However, when running fortran-checking tests with GNU fortran, each
would fail due to the post-run addition of the GFC symbol added in
v2.68-97-gbd962ac.
* tests/local.at (AT_CHECK_ENV): Add GFC to the list of symbols
that we ignore in pre-/post-run environment diffs.