bpo-32989: IDLE - fix bad editor call of pyparse method (GH-5968)

Fix comments and add tests for editor newline_and_indent_event method.
Remove unused None default for function parameter of pyparse find_good_parse_start method
and code triggered by that default.

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
diff --git a/Lib/idlelib/pyparse.py b/Lib/idlelib/pyparse.py
index feb57cb..9fa2010 100644
--- a/Lib/idlelib/pyparse.py
+++ b/Lib/idlelib/pyparse.py
@@ -133,8 +133,7 @@
         self.code = s
         self.study_level = 0
 
-    def find_good_parse_start(self, is_char_in_string=None,
-                              _synchre=_synchre):
+    def find_good_parse_start(self, is_char_in_string, _synchre=_synchre):
         """
         Return index of a good place to begin parsing, as close to the
         end of the string as possible.  This will be the start of some
@@ -149,10 +148,6 @@
         """
         code, pos = self.code, None
 
-        if not is_char_in_string:
-            # no clue -- make the caller pass everything
-            return None
-
         # Peek back from the end for a good place to start,
         # but don't try too often; pos will be left None, or
         # bumped to a legitimate synch point.