scripts: test.py/bench.py: Added -e/--exec shortform flag

Why not, -e/--exec seems useful/general purpose enough to deserve a
shortform flag. Especially since much of our testing involves emulation.

The only risk of conflicts is with -e/--error-* in other scripts, but
the _whole point_ of test.py is to error on failure, so I don't think
this will be an issue.

Note that -E may be more useful for environment variables in the future.

I feel like -e/--exec was more common in other programs, but I've only
found sed -e and perl -e so far. Most programs stick to -c/--command
(bash, python) which would conflict with -c/--compile here.

scripts/test.py

@@ -1696,7 +1696,7 @@ if __name__ == "__main__":
             help="Path to the gdb executable, may include flags. "
                 "Defaults to %r." % GDB_PATH)
     test_parser.add_argument(
-            '--exec',
+            '-e', '--exec',
             type=lambda e: e.split(),
             help="Run under another executable.")
     test_parser.add_argument(