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>

Author: mknyszek

src/runtime/pprof/mprof_test.go

@@ -86,6 +86,17 @@ func TestMemoryProfiler(t *testing.T) {
 
 	runtime.GC() // materialize stats
 
+	// TODO(mknyszek): Fix #45315 and remove this extra call.
+	//
+	// Unfortunately, it's possible for the sweep termination condition
+	// to flap, so with just one runtime.GC call, a freed object could be
+	// missed, leading this test to fail. A second call reduces the chance
+	// of this happening to zero, because sweeping actually has to finish
+	// to move on to the next GC, during which nothing will happen.
+	//
+	// See #46500 for more details.
+	runtime.GC()
+
 	memoryProfilerRun++
 
 	tests := []struct {