runtime/pprof: deflake TestMorestack more

Apparently, TestMorestack is still flaky on darwin/arm64 builder
after CL 307730. Let it spend more time in copying the stack.
With this CL, on my Apple M1 machine it passes reliably in short
mode for 1000 runs, and reliably gets 250+ samples in the 5-second
interval in long mode.

May fix #46755.

Change-Id: I07b36c1cf63ad35f7820e1f8e837e29376a37b2a
Reviewed-on: https://go-review.googlesource.com/c/go/+/329869


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>