[3.10] bpo-44368: Improve syntax errors with invalid as pattern targets (GH-26632) (GH-26792)

(cherry picked from commit 05073036dcecefc00b0c3e7397601809da41e2f1)

Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>
diff --git a/Lib/test/test_syntax.py b/Lib/test/test_syntax.py
index 72e4ab1..e0d0445 100644
--- a/Lib/test/test_syntax.py
+++ b/Lib/test/test_syntax.py
@@ -1226,6 +1226,20 @@
     >>> import ä £
     Traceback (most recent call last):
     SyntaxError: invalid character '£' (U+00A3)
+
+  Invalid pattern matching constructs:
+
+    >>> match ...:
+    ...   case 42 as _:
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: cannot use '_' as a target
+
+    >>> match ...:
+    ...   case 42 as 1+2+4:
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: invalid pattern target
 """
 
 import re