Add trivial tests for gnt-* cli

While testing some other stuff, I realised that the gnt-* commands
could be broken (as in, the script fails with syntax errors), but make
check doesn't detect it. Since we have shelltest, we can now add
trivial tests for this case.

One downside is that starting the scripts seems to be much slower
than the htools binaries, so we can't add as many tests.

The other downside is that shelltest is now required for all
development work, but I think this is a small disadvantage compared to
the increased testing possibilities.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>