ffi 2.0: xfail parser test for Foo :: #jni_class(...) { ... }

Today's parser doesn't recognize #jni_class as a hash directive
after `::`, so it falls through to expression parsing and errors
at the `#` token. Step 2.1 extends parseConstBinding to accept
the directive (opaque on empty body) and re-snapshots this file
to green.

tests/expected/ffi-jni-class-01-empty.exit

@@ -0,0 +1 @@
+1

tests/expected/ffi-jni-class-01-empty.txt

@@ -0,0 +1 @@
+/Users/agra/projects/sx/examples/ffi-jni-class-01-empty.sx:13:8: error: unexpected token in expression