cmd/go/testdata/script: fix flaky test_fuzz_fuzztime test

Previously, the test would panic if the short timeout
was reached before fuzzing began. Increasing the
timeout should stop this test from being flaky.

Fixes #49046

Change-Id: Iaa0b3b3e8ea29d9a42ab5fc1c801fc73fffe1675
Reviewed-on: https://go-review.googlesource.com/c/go/+/358055


Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>