fix[test]: fix failure in grammar fuzzing (#3892)

fixes a fuzz failure, which is that the strategy can put utf-8
characters in strings (which we reject at compile-time). the fix here is
to just create programs with ascii. this bug was apparently introduced
in 176e7f7 due to the new alphabet
parameter passed to the grammar strategy.

tests/functional/grammar/test_grammar.py

@@ -44,7 +44,7 @@ def fix_terminal(terminal: str) -> bool:
     return terminal
 
 
-ALLOWED_CHARS = st.characters(codec="utf-8", min_codepoint=1)
+ALLOWED_CHARS = st.characters(codec="ascii", min_codepoint=1)
 
 
 # With help from hyposmith