test: really force heap profile update in heapsampling.go

The heap profile is only guaranteed to be up-to-date after two GC
cycles, so force two GCs instead of just one.

Updates #13098.

Change-Id: I4fb9287b698f4a3b90b8af9fc6a2efb3b082bfe5
Reviewed-on: https://go-review.googlesource.com/16848Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

test/heapsampling.go

@@ -92,6 +92,9 @@ func checkValue(fname string, ln int, name string, want, got int64) {
 
 func getMemProfileRecords() []runtime.MemProfileRecord {
 	// Force the runtime to update the object and byte counts.
+	// This can take up to two GC cycles to get a complete
+	// snapshot of the current point in time.
+	runtime.GC()
 	runtime.GC()
 
 	// Find out how many records there are (MemProfile(nil, true)),