Improve captureTestingT helper

This helper is used to capture the testing.TB interface, and compare
the log output with the expected output.

This is useful for testing and refactoring purposes.

This commit improves the helper by displaying:
- the captured and expected outputs on a newline.
- the special characters in the captured output, such as newlines and tabs.

Both help with readability.

Here is an example of the output before and after the change:

Before:

    assertions_test.go:3422: Logged Error: Should be in error chain
        expected: *assert.customError
        in chain: "EOF" (*errors.errorString)
    assertions_test.go:3422: Should log Error: Should be in error chain:
        expected: *assert.customError
        in chain: "EOF" (*errors.errorString)

After:
    assertions_test.go:3394: Recorded Error: "Should be in error chain:\nexpected: *assert.customError\nin chain: \"EOF\" (*errors.errorString)\n"
    assertions_test.go:3394: Expected Error: "Should be in error chain\nexpected: *assert.customError\nin chain: \"EOF\" (*errors.errorString)"

The new format helps to identify the differences:
- the missing colon after "Should be in error chain"
- the extra newline in the captured output

Note: I spent 10 minutes on this change, because I lost 10 minutes in
finding the differences between the captured and expected output on a
refactoring I was doing.

Author: ccoVeille

assert/assertions_test.go

@@ -3675,23 +3675,23 @@ func (ctt *captureTestingT) checkResultAndErrMsg(t *testing.T, expectedRes, res
 	contents := parseLabeledOutput(ctt.msg)
 	if res == true {
 		if contents != nil {
-			t.Errorf("Should not log an error")
+			t.Errorf("Should not log an error. Log output: %q", ctt.msg)
 		}
 		return
 	}
 	if contents == nil {
-		t.Errorf("Should log an error. Log output: %v", ctt.msg)
+		t.Errorf("Should log an error. Log output: %q", ctt.msg)
 		return
 	}
 	for _, content := range contents {
 		if content.label == "Error" {
 			if expectedErrMsg == content.content {
 				return
 			}
-			t.Errorf("Logged Error: %v", content.content)
+			t.Errorf("Recorded Error: %q", content.content)
 		}
 	}
-	t.Errorf("Should log Error: %v", expectedErrMsg)
+	t.Errorf("Expected Error: %q", expectedErrMsg)
 }
 
 func TestErrorIs(t *testing.T) {