fix[test]: fix test in grammar fuzzer (#4150)

hypothesis `6.103.1` generates examples like

```
Falsifying example: test_grammar_bruteforce(
    code='"""\r""" \n""""""',
)
```

which trivially raise an exception in asttokens. filter out `\r`
characters when generating source codes.

tests/functional/grammar/test_grammar.py

@@ -39,7 +39,7 @@ def test_basic_grammar_empty():
 
 def fix_terminal(terminal: str) -> str:
     # these throw exceptions in the grammar
-    for bad in ("\x00", "\\ ", "\x0c"):
+    for bad in ("\x00", "\\ ", "\x0c", "\x0d"):
         terminal = terminal.replace(bad, " ")
     return terminal