bpo-33204: IDLE - revise and extend colorizer test. (GH-6347)

Followup to primary PR for the issue, GH-6344.
(cherry picked from commit 55966f3a)
Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

Lib/idlelib/colorizer.py

@@ -262,9 +262,12 @@ def _color_delegator(parent):  # htest #
     top.title("Test ColorDelegator")
     x, y = map(int, parent.geometry().split('+')[1:])
     top.geometry("700x250+%d+%d" % (x + 20, y + 175))
-    source = ("# Following has syntax errors\n"
-        "if True: then int 1\nelif False: print 0\nelse: float(None)\n"
-        "if iF + If + IF: 'keywork matching must respect case'\n"
+    source = (
+        "if True: int ('1') # keyword, builtin, string, comment\n"
+        "elif False: print(0)\n"
+        "else: float(None)\n"
+        "if iF + If + IF: 'keyword matching must respect case'\n"
+        "if'': x or''  # valid string-keyword no-space combinations\n"
         "# All valid prefixes for unicode and byte strings should be colored.\n"
         "'x', '''x''', \"x\", \"\"\"x\"\"\"\n"
         "r'x', u'x', R'x', U'x', f'x', F'x'\n"

Misc/NEWS.d/next/IDLE/2018-04-02-00-28-13.bpo-33204.NBsuIv.rst

 IDLE: consistently color invalid string prefixes. A 'u' string prefix cannot
 be paired with either 'r' or 'f'. Consistently color as much of the prefix,
-starting at the right, as is valid.
+starting at the right, as is valid. Revise and extend colorizer test.