libgo: uncomment trace.Stop() call in testing package

Fix up the testing package to insure that execution traces
    work properly (e.g. "-test.trace=<XXX>" command line option). The
    call to stop tracing and emit the output file was stubbed out.
    
    Reviewed-on: https://go-review.googlesource.com/128275

From-SVN: r263363

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-8997a3afcc746824cb70b48b32d9c86b4814807d
+274c88df4d6f9360dcd657b6e069a3b5a1d37a90
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/testing/testing.go

@@ -1159,7 +1159,7 @@ func (m *M) writeProfiles() {
 		m.deps.StopCPUProfile() // flushes profile to disk
 	}
 	if *traceFile != "" {
-		// trace.Stop() // flushes trace to disk
+		trace.Stop() // flushes trace to disk
 	}
 	if *memProfile != "" {
 		f, err := os.Create(toOutputDir(*memProfile))