runtime/pprof: fix retry logic introduced by #858cd8d

The previous attempt to fix this flake was incorrect in that it
examined the vmmap output rather than the detailed error
output for the 'resource shortage' message that triggers the
retry, and hence failed to retry. This PR looks at the
detailed error output.

Fixes golang#62352

Change-Id: I4218b187528a95842556dc1ea27947ffcbfbc497
Reviewed-on: https://go-review.googlesource.com/c/go/+/558575
Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/pprof/vminfo_darwin_test.go

@@ -79,9 +79,9 @@ func useVMMap(t *testing.T) (hi, lo uint64, retryable bool, err error) {
 		t.Logf("vmmap output: %s", out)
 		if ee, ok := cmdErr.(*exec.ExitError); ok && len(ee.Stderr) > 0 {
 			t.Logf("%v: %v\n%s", cmd, cmdErr, ee.Stderr)
+			retryable = bytes.Contains(ee.Stderr, []byte("resource shortage"))
 		}
-		retryable = bytes.Contains(out, []byte("resource shortage"))
-		t.Logf("%v: %v", cmd, cmdErr)
+		t.Logf("%v: %v\n", cmd, cmdErr)
 		if retryable {
 			return 0, 0, true, cmdErr
 		}