gh-156955: Cover line terminator quoting in the csv writer tests

Apply the reviewer's suggestion on GH-156956: exercise csv.writer with a
field that embeds the line terminator, over terminators spanning the
latin-1, BMP and non-BMP string kinds.

Add two more tests for the paths the optimization introduced but nothing
pinned:

  * test_write_lineterminator_quoting -- every character of a
    multi-character terminator forces quoting, not just the last, and
    characters bracketing the terminator in code point order do not.
  * test_write_empty_lineterminator -- an empty terminator has no
    characters, so it separates nothing and quotes nothing, including
    '\0'.

All three pass against Modules/_csv.c as it stood before the
optimization, so they pin existing csv.writer semantics rather than new
behavior.

Author: brittanyrey

Lib/test/test_csv.py

@@ -260,17 +260,44 @@ def test_write_escape(self):
                          escapechar='\\', quoting=csv.QUOTE_MINIMAL)
 
     def test_write_lineterminator(self):
-        for lineterminator in '\r\n', '\n', '\r', '!@#', '\0':
+        for lineterminator in ('\r\n', '\n', '\r', '!@#', '\0', '\x85',
+                               '\u2028', '\U0001f600'):
             with self.subTest(lineterminator=lineterminator):
                 with StringIO() as sio:
                     writer = csv.writer(sio, lineterminator=lineterminator)
                     writer.writerow(['a', 'b'])
                     writer.writerow([1, 2])
                     writer.writerow(['\r', '\n'])
+                    writer.writerow([f'a{lineterminator[-1]}b', 'c'])
                     self.assertEqual(sio.getvalue(),
                                      f'a,b{lineterminator}'
                                      f'1,2{lineterminator}'
-                                     f'"\r","\n"{lineterminator}')
+                                     f'"\r","\n"{lineterminator}'
+                                     f'"a{lineterminator[-1]}b",c{lineterminator}')
+
+    def test_write_lineterminator_quoting(self):
+        # Every character of the line terminator forces quoting, not just the
+        # last one, and no other character does.  Each terminator is paired
+        # with characters that bracket it in code point order, to pin that
+        # boundary.
+        for lineterminator, plain in ('!@#', ' ?A'), ('\u2028', '\u2027\u2029'):
+            with self.subTest(lineterminator=lineterminator):
+                for c in lineterminator:
+                    self._write_test([f'a{c}b', 'c'], f'"a{c}b",c',
+                                     lineterminator=lineterminator)
+                self._write_test([f'a{plain}b', 'c'], f'a{plain}b,c',
+                                 lineterminator=lineterminator)
+
+    def test_write_empty_lineterminator(self):
+        # An empty line terminator separates nothing and, having no characters
+        # of its own, forces no quoting -- not even of '\0', the lowest code
+        # point.  '\r' and '\n' are quoted whatever the terminator is.
+        with StringIO() as sio:
+            writer = csv.writer(sio, lineterminator='')
+            writer.writerow(['a', 'b'])
+            writer.writerow(['\0', 'c'])
+            writer.writerow(['\r', '\n'])
+            self.assertEqual(sio.getvalue(), 'a,b\0,c"\r","\n"')
 
     def test_write_iterable(self):
         self._write_test(iter(['a', 1, 'p,q']), 'a,1,"p,q"')