This patch fixes the problem by adding a new rule booleat_test.
This makes the grammar clearer and less conflicting.

Additionally, fixing %prec in this grammar branch:

-        | boolean_test IS NULL_SYM %prec PREC_BELOW_NOT
+        | boolean_test IS NULL_SYM %prec IS

to have consistently "%prec IS" in all grammar branches starting
with "boolean_test IS ...".
It's not clear why these three rules needed different %prec before the fix:

- boolean_test IS TRUE
- boolean_test IS UNKNOWN
- boolean_test IS NULL