pkg/jsonmessage: Wrap test failure messages so control characters align

When logging the failure of a case which involves control characters add line
feeds and spacing such that the expected and actual strings are aligned in the
same columns making comparisons easier.

Turns:
	jsonmessage_test.go:156: Expected "\x1b[2K\rstatus      1 B\r", got "\x1b[1K\x1b[K\rstatus      1 B\r"
Into:
	jsonmessage_test.go:156:
		Expected "\x1b[2K\rstatus      1 B\r"
		     got "\x1b[1K\x1b[K\rstatus      1 B\r"

Signed-off-by: Ian Campbell <ian.campbell@docker.com>

pkg/jsonmessage/jsonmessage_test.go

@@ -144,7 +144,7 @@ func TestJSONMessageDisplay(t *testing.T) {
 			t.Fatal(err)
 		}
 		if data.String() != expectedMessages[0] {
-			t.Fatalf("Expected %q, got %q", expectedMessages[0], data.String())
+			t.Fatalf("Expected %q,got %q", expectedMessages[0], data.String())
 		}
 		// With terminal
 		data = bytes.NewBuffer([]byte{})
@@ -152,7 +152,7 @@ func TestJSONMessageDisplay(t *testing.T) {
 			t.Fatal(err)
 		}
 		if data.String() != expectedMessages[1] {
-			t.Fatalf("Expected %q, got %q", expectedMessages[1], data.String())
+			t.Fatalf("\nExpected %q\n     got %q", expectedMessages[1], data.String())
 		}
 	}
 }
@@ -238,7 +238,7 @@ func TestDisplayJSONMessagesStream(t *testing.T) {
 			t.Fatal(err)
 		}
 		if data.String() != expectedMessages[1] {
-			t.Fatalf("Expected an %q, got %q", expectedMessages[1], data.String())
+			t.Fatalf("\nExpected %q\n     got %q", expectedMessages[1], data.String())
 		}
 	}