Improve parser test error messages

Summary:
This diff breaks up passing and failing tests into helpers: `parse_ast` and `parse_ast_should_fail` respectively.

The big win is that some tests forgot to put `EXPECT_THAT(diagnostics, IsEmpty())` which shows the parse error if the test fails. This dif means that all tests should now do the right thing.

Reviewed By: rmakheja

Differential Revision: D69095813

fbshipit-source-id: 25f1d8de193ecaf802e82e9c39fad4db861b8b62