pythongh-57539: Add tests for LocaleTextCalendar.formatweekday

This patch has already been applied to `main` branch (via pythongh-93468), but
with wrong copyright. After merging this commit to `main`, git `Author`
metadata will be updated to the original author.

Lib/test/test_calendar.py

@@ -605,6 +605,16 @@ def test_locale_calendars(self):
         new_october = calendar.TextCalendar().formatmonthname(2010, 10, 10)
         self.assertEqual(old_october, new_october)
 
+    def test_locale_calendar_formatweekday(self):
+        # formatweekday uses different day names based on the available width.
+        cal = calendar.LocaleTextCalendar(locale='en_US')
+        # For short widths, a centered, abbreviated name is used.
+        self.assertEqual(cal.formatweekday(0, 5), " Mon ")
+        # For really short widths, even the abbreviated name is truncated.
+        self.assertEqual(cal.formatweekday(0, 2), "Mo")
+        # For long widths, the full day name is used.
+        self.assertEqual(cal.formatweekday(0, 10), "  Monday  ")
+
     def test_locale_html_calendar_custom_css_class_month_name(self):
         try:
             cal = calendar.LocaleHTMLCalendar(locale='')