net: make use of testing.Errorf instead of testing.Fatalf in non-test function goroutines

See testing.FailNow for further information.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/75900043
diff --git a/src/pkg/net/timeout_test.go b/src/pkg/net/timeout_test.go
index 35d427a..40b5068 100644
--- a/src/pkg/net/timeout_test.go
+++ b/src/pkg/net/timeout_test.go
@@ -348,7 +348,8 @@
 	go func() {
 		c, err := ln.Accept()
 		if err != nil {
-			t.Fatalf("Accept: %v", err)
+			t.Errorf("Accept: %v", err)
+			return
 		}
 		defer c.Close()
 		lnquit <- true
@@ -536,7 +537,8 @@
 	go func() {
 		c, err := ln.Accept()
 		if err != nil {
-			t.Fatalf("Accept: %v", err)
+			t.Errorf("Accept: %v", err)
+			return
 		}
 		defer c.Close()
 		n, err := c.Write([]byte(msg))
@@ -574,7 +576,8 @@
 	go func() {
 		c, err := ln.Accept()
 		if err != nil {
-			t.Fatalf("Accept: %v", err)
+			t.Errorf("Accept: %v", err)
+			return
 		}
 		defer c.Close()
 		c.SetWriteDeadline(time.Now().Add(-5 * time.Second)) // in the past
@@ -610,7 +613,8 @@
 	go func() {
 		c, err := Dial("tcp", ln.Addr().String())
 		if err != nil {
-			t.Fatalf("Dial: %v", err)
+			t.Errorf("Dial: %v", err)
+			return
 		}
 		defer c.Close()
 		var buf [1]byte
@@ -669,7 +673,8 @@
 		s, err := ln.Accept()
 		connected <- true
 		if err != nil {
-			t.Fatalf("ln.Accept: %v", err)
+			t.Errorf("ln.Accept: %v", err)
+			return
 		}
 		defer s.Close()
 		s.SetDeadline(time.Now().Add(time.Hour))