Probe for GNU parallel more accurately

Probe for GNU parallel more accurately

moreutils parallel isn't compatible with the command-line syntax
used here. Because it doesn't implement GNU-style --help and exits 1
when that option is given, this test was correctly skipped when
using moreutils parallel, but only by mistake.

moreutils parallel might conceivably gain --help and --version in
future, but hopefully nothing incompatible with GNU parallel is going
to gain a --gnu option.

Also use the --gnu option to force the new command-line semantics;
some versions optionally supported an incompatible command-line syntax
taken from moreutils parallel.

Signed-off-by: Simon McVittie <smcv@debian.org>
Closes: #232
Approved by: cgwalters