test: Make run.go's errorcheck behave like testlib.

testlib will complain about any unmatched errors left in errorchecks while run.go will not.

Fixes #4141.

R=golang-dev, minux.ma, remyoudompheng, rsc
CC=golang-dev
https://golang.org/cl/6614060
diff --git a/test/run.go b/test/run.go
index b6437ee..b79323d 100644
--- a/test/run.go
+++ b/test/run.go
@@ -522,7 +522,9 @@
 		}
 		if strings.HasPrefix(line, "\t") {
 			out[len(out)-1] += "\n" + line
-		} else {
+		} else if strings.HasPrefix(line, "go tool") {
+			continue
+		} else if strings.TrimSpace(line) != "" {
 			out = append(out, line)
 		}
 	}
@@ -553,6 +555,13 @@
 		}
 	}
 
+	if len(out) > 0 {
+		errs = append(errs, fmt.Errorf("Unmatched Errors:"))
+		for _, errLine := range out {
+			errs = append(errs, fmt.Errorf("%s", errLine))
+		}
+	}
+
 	if len(errs) == 0 {
 		return nil
 	}