gotest: increase the test timeout

The default test timeout duration of the gc compiler is 10 minutes,
and the current default timeout duration of gofrontend is 240 seconds,
which is not long enough for some big tests. This CL changes it to
600s, so that all tests have enough time to complete.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/229657