Fixed underflow issue with truncating test/bench -C/--context

There was no check on context > stdout, so requesting more context than
was actually printed by the test could result in a negative value.
Python "helpfully" interpreted this as a negative index, resulting in
somewhat random context lengths.

This, combined with my tendency to just default to a large number like
--context=100, led to me thinking a test was printing much less than it
actually was...

Don't get me wrong, I love Python, and I think Python's negative indices
are a clever way to add flexibility to slice notation, but the
value-dependent semantics are a pretty unfortunate footgun...

scripts/test.py

@@ -1306,7 +1306,7 @@ def run(runner, test_ids=[], **args):
             stdout = failure.stdout
             if failure.assert_ is not None:
                 stdout = stdout[:-1]
-            for line in stdout[len(stdout)-args.get('context', 5):]:
+            for line in stdout[max(len(stdout)-args.get('context', 5), 0):]:
                 sys.stdout.write(line)
 
         if failure.assert_ is not None: