Make sure we reject everything in src/test/ui/parser/

[Centril]

We'd like the grammar to reject everything inside https://github.com/rust-lang/rust/tree/master/src/test/ui/parser since those are parse-fail tests.

[eddyb]

We might want to look for hints like // compile-flags: with -Z parse-only on the same line, accompanied by ERROR comments and/or a stderr file, instead of hardcoding a directory.

[Centril]

Yeah that might be more robust, but it also relies on -Z parse-only remaining the way in compile_test so you'll have to make sure that the rest of the compiler team don't sweep that out from under us without prior notice ;)

[petrochenkov]

All -Z parse-only are indeed in ui/parser currently, but ui/parser also contains compile-pass tests and some non--Z parse-only tests doing parsing-during-expansion, so #11 (comment) is certainly more robust.