[3.12] gh-85012: Properly reset msgctxt when compiling messages with msgfmt (GH-130525) (GH-131206)

Add also human-readable snapshots for tests.
(cherry picked from commit 7ea6e88eb4)

Co-authored-by: Tomas R <tomas.roun8@gmail.com>

Tools/i18n/msgfmt.py

@@ -149,6 +149,7 @@ def make(filename, outfile):
         elif l.startswith('msgid') and not l.startswith('msgid_plural'):
             if section == STR:
                 add(msgctxt, msgid, msgstr, fuzzy)
+                msgctxt = None
                 if not msgid:
                     # See whether there is an encoding declaration
                     p = HeaderParser()