Adjust test involving FixtureLookupErrorRepr

I added a starting 'E' to the expected error messages.

The tests were still passing after the previous patch but I think it's
better to have stricter tests.

testing/acceptance_test.py

@@ -376,7 +376,7 @@ class TestGeneralUsage:
         res = testdir.runpytest(p)
         res.stdout.fnmatch_lines([
             "*source code not available*",
-            "*fixture 'invalid_fixture' not found",
+            "E*fixture 'invalid_fixture' not found",
         ])
 
     def test_plugins_given_as_strings(self, tmpdir, monkeypatch):

testing/test_capture.py

@@ -416,9 +416,9 @@ class TestCaptureFixture:
         result = testdir.runpytest(p)
         result.stdout.fnmatch_lines([
             "*ERROR*setup*test_one*",
-            "*capsys*capfd*same*time*",
+            "E*capsys*capfd*same*time*",
             "*ERROR*setup*test_two*",
-            "*capsys*capfd*same*time*",
+            "E*capsys*capfd*same*time*",
             "*2 error*"])
 
     @pytest.mark.parametrize("method", ["sys", "fd"])