Re-enable compile-fail tests

[dtolnay]

Disabled temporarily in #1512 due to libserde now appearing in the nightly sysroot and causing a conflict. We will need to figure out how else to run these tests.

[dtolnay]

I think the solution will involve factoring out the test runner from https://github.com/dtolnay/proc-macro-workshop and dropping compiletest_rs in favor of that. It is a more robust approach based on Cargo rather than direct rustc invocations. I have found that it works very reliably and in particular would not have this class of problem.

Upstream issue: dtolnay/proc-macro-workshop#14

[oli-obk]

Here's the clippy workaround: rust-lang/rust-clippy#4017