test: name the literal-character test for what it exercises

`test_a_unicode_escape_becomes_its_character` fed a literal e-acute rather
than a backslash-u escape spelling one, so it passed against the unfixed
writer and proved nothing about the escape form its name claimed.
`TestTheWriterResolvesUnicodeEscapes` already covers both escape forms, so
rename this one to say what it does test -- that a non-ASCII character
written literally survives the trip into a heredoc body -- and point the
newer class's docstring at it rather than at "the earlier test".

Author: livingstaccato

test/unit/test_template_spans.py

@@ -95,7 +95,7 @@ def _body(self, value: str) -> str:
     def test_a_tab_becomes_a_tab(self):
         self.assertEqual(self._body(r'"a\tb\n"'), "a = <<EOF\na\tb\nEOF\n")
 
-    def test_a_unicode_escape_becomes_its_character(self):
+    def test_a_literal_non_ascii_character_survives(self):
         self.assertEqual(self._body(r'"café\n"'), "a = <<EOF\ncafé\nEOF\n")
 
     def test_a_wide_unicode_escape_becomes_its_character(self):
@@ -263,7 +263,11 @@ def test_characters_left_unescaped_are_legal_as_themselves(self):
 
 
 class TestTheWriterResolvesUnicodeEscapes(TestCase):
-    r"""The earlier test used a literal e-acute, which passes without the fix."""
+    r"""`\uNNNN` and `\UNNNNNNNN`, written as escapes rather than as the character.
+
+    `TestTheWriterResolvesEveryEscape` covers a literal non-ASCII character, which
+    the writer passed through even before it resolved the escape forms.
+    """
 
     def test_a_bmp_escape_becomes_its_character(self):
         source = '"' + chr(92) + "u00e9" + chr(92) + "n" + '"'