test-web: redirect stdio to make test work standalone

test-web: redirect stdio to make test work standalone

Python (at least 3) behaves differently if somehow
stdio is connected to the terminal, and external
SIGTERM then doesn't cause the same behaviour as
actually pressing Ctrl-C. Redirect the output to
a file to make the test run standalone.

Print the file if any errors occur so that if there's
anything interesting in there, we can see it.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>