Don't use vec[] in unit test

Only need to test UnsupportedTypeException (not IncorrectTypeException), and
vec[] doesn't work in 3.12 or 3.15.

Author: fredemmott

tests/TypeStructureTest.php

@@ -267,8 +267,7 @@ public function testUnsupportedType(): void {
     $this->expectException(UnsupportedTypeException::class);
     TypeAssert::matchesTypeStructure(
        type_structure(C::class, 'TVec'),
-       /* HH_IGNORE_ERROR[0000] cannot use experimental feature */
-       vec[1, 2, 3],
+       null,
     );
   }
 }