bpo-38041: Refine IDLE Shell restart lines. (GH-15709)

Restart lines now always start with '=' and never end with ' ' and fill the width of the window unless that would require ending with ' ', which could be wrapped by itself and possible confusing the user.

Lib/idlelib/idle_test/test_pyshell.py

@@ -7,6 +7,28 @@ from test.support import requires
 from tkinter import Tk
 
 
+class FunctionTest(unittest.TestCase):
+    # Test stand-alone module level non-gui functions.
+
+    def test_restart_line_wide(self):
+        eq = self.assertEqual
+        for file, mul, extra in (('', 22, ''), ('finame', 21, '=')):
+            width = 60
+            bar = mul * '='
+            with self.subTest(file=file, bar=bar):
+                file = file or 'Shell'
+                line = pyshell.restart_line(width, file)
+                eq(len(line), width)
+                eq(line, f"{bar+extra} RESTART: {file} {bar}")
+
+    def test_restart_line_narrow(self):
+        expect, taglen = "= RESTART: Shell", 16
+        for width in (taglen-1, taglen, taglen+1):
+            with self.subTest(width=width):
+                self.assertEqual(pyshell.restart_line(width, ''), expect)
+        self.assertEqual(pyshell.restart_line(taglen+2, ''), expect+' =')
+
+
 class PyShellFileListTest(unittest.TestCase):
 
     @classmethod