Close #22756: Improve the test output for some assertEqual tests.

These tests were undebuggable as written, and there's no testing fallacy
involved in using the method we're testing to test the output of that method,
so switch to that.

Lib/unittest/test/test_case.py

@@ -1075,10 +1075,7 @@ test case
         except self.failureException as e:
             # need to remove the first line of the error message
             error = str(e).split('\n', 1)[1]
-
-            # no fair testing ourself with ourself, and assertEqual is used for strings
-            # so can't use assertEqual either. Just use assertTrue.
-            self.assertTrue(sample_text_error == error)
+            self.assertEqual(sample_text_error, error)
 
     def testAssertEqualSingleLine(self):
         sample_text = "laden swallows fly slowly"
@@ -1092,8 +1089,9 @@ test case
         try:
             self.assertEqual(sample_text, revised_sample_text)
         except self.failureException as e:
+            # need to remove the first line of the error message
             error = str(e).split('\n', 1)[1]
-            self.assertTrue(sample_text_error == error)
+            self.assertEqual(sample_text_error, error)
 
     def testAssertIsNone(self):
         self.assertIsNone(None)