[lldb] Replace assertTrue(foo in bar) with assertIn(foo, bar)

The benefit of using assertIn is an improved error message when the
assertion fails:

  AssertionError: False is not True

becomes

  AssertionError: 'have ints 5 20 20 5' not found in '""'

Author: JDevlieghere

lldb/test/API/macosx/skinny-corefile/TestSkinnyCorefile.py

@@ -140,7 +140,7 @@ def test_lc_note(self):
         self.assertEqual(to_be_removed_dirty_data.GetValueAsUnsigned(), 20)
 
         present_heap_buf = f0.FindVariable("present_heap_buf")
-        self.assertTrue("have ints 5 20 20 5" in present_heap_buf.GetSummary())
+        self.assertIn("have ints 5 20 20 5", present_heap_buf.GetSummary())
 
 
         # f1 is to_be_removed() in libto-be-removed.dylib