runtime/pprof: call runtime.GC twice in memory profile test

runtime/pprof: call runtime.GC twice in memory profile test

This change fixes #46500 by working around #45315 which may cause freed
objects to get missed in the heap profile published for the test.

By calling runtime.GC one more time this change ensures that all freed
objects are accounted for.

Fixes #46500.

Change-Id: Iedcd0b37dbaffa688b0ff8631a8b79f7a1169634
Reviewed-on: https://go-review.googlesource.com/c/go/+/333549
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>