ffi 2.6 xfail: #desc("(Sig)Ret") per-method JNI descriptor override

`weirdMethod :: (self: *Self) -> s32 #desc("()I");` should parse,
today's 2.5 parser expects `;` immediately after the return type
and errors at the `#desc` token. The make-green follow-up adds a
`hash_desc` lexer token and threads an optional `desc_override`
field through `JniMethodDecl`.

tests/expected/ffi-jni-class-06-desc.exit

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

tests/expected/ffi-jni-class-06-desc.txt

@@ -0,0 +1 @@
+/Users/agra/projects/sx/examples/ffi-jni-class-06-desc.sx:14:41: error: expected ';'