Change syntax_error test for consistent output

Python 3.11 changes the string representation of the
SyntaxError triggered by this test - it now says "expected '('"
instead of just "invalid syntax". This changes the test to use
a different error (incomplete `for` loop) which still has just
"invalid syntax" as its description in Python 3.11. This is the
same 'bad code' used in the similar `test_stdin_syntaxerror` in
the unit tests.

Signed-off-by: Adam Williamson <[email protected]>

Author: AdamWill

tests/functional/s/syntax/syntax_error.py

@@ -1 +1 @@
-def toto # [syntax-error]
+for # [syntax-error]

tests/functional/s/syntax/syntax_error.txt

@@ -1 +1 @@
-syntax-error:1:10:None:None::invalid syntax (<unknown>, line 1):UNDEFINED
+syntax-error:1:5:None:None::invalid syntax (<unknown>, line 1):UNDEFINED