cmd/go: do not expect a specific timing in test_fuzz_minimize

cmd/go: do not expect a specific timing in test_fuzz_minimize

Empirically, it may take more than 3 seconds for minimization to begin.
If that is the case, the "elapsed:" message may start at 4s or higher.

Fixes #48870

Change-Id: I6bb4367b61377454b0af664d80232fdb46e7c245
Reviewed-on: https://go-review.googlesource.com/c/go/+/354794
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>