Add syntax error message tests

Author: bswck

Lib/test/test_syntax.py

@@ -370,12 +370,6 @@
 Traceback (most recent call last):
 SyntaxError: invalid syntax
 
->>> match ...:
-...     case {**rest, "key": value}:
-...        ...
-Traceback (most recent call last):
-SyntaxError: invalid syntax
-
 >>> match ...:
 ...     case {**_}:
 ...        ...
@@ -2240,7 +2234,7 @@
     Traceback (most recent call last):
     SyntaxError: invalid character '£' (U+00A3)
 
-  Invalid pattern matching constructs:
+Invalid pattern matching constructs:
 
     >>> match ...:
     ...   case 42 as _:
@@ -2302,6 +2296,18 @@
     Traceback (most recent call last):
     SyntaxError: positional patterns follow keyword patterns
 
+    >>> match ...:
+    ...   case {**double_star, "spam": "eggs"}:
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: double star pattern must be the last (right-most) subpattern in the mapping pattern
+
+    >>> match ...:
+    ...   case {"foo": 1, **double_star, "spam": "eggs"}:
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: double star pattern must be the last (right-most) subpattern in the mapping pattern
+
 Uses of the star operator which should fail:
 
 A[:*b]