test: fix test.py command line options processing

nodejs/node#11086 had introduced a regression
that broke command line options processing for tools/test.py.

Basically, it made tools/test.py discard the command line argument that
would be passed after `--abort-on-timeout`. For instance, when running:

```
$ python tools/test.py --abort-on-timeout path/to/some-test
```

all tests would be run because the last command line argument
(`/path/to/some-test`) would be discarded.

This change fixes this regression.

Refs: nodejs/node#11086
PR-URL: nodejs/node#11153
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

tools/test.py

@@ -1399,7 +1399,7 @@ def BuildOptions():
       default=1, type="int")
   result.add_option('--abort-on-timeout',
       help='Send SIGABRT instead of SIGTERM to kill processes that time out',
-      default=False, dest="abort_on_timeout")
+      default=False, action="store_true", dest="abort_on_timeout")
   return result