Import resolution regression when test has the same name as extern macro

[dtolnay]

I am seeing some breakage in indoc from #52890. It minimizes to:

#![feature(use_extern_macros)]

#[test]
fn unimplemented() {}

fn main() {
    unimplemented!();
}

$ cargo +nightly-2018-08-02 test
   Compiling repro v0.0.0
    Finished dev [unoptimized + debuginfo] target(s) in 0.56s
     Running target/debug/deps/repro-1d729be478cd3ccf

running 1 test
test unimplemented ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

$ cargo +nightly-2018-08-03 test
   Compiling repro v0.0.0
error: cannot determine resolution for the macro `unimplemented`
 --> src/main.rs:7:5
  |
7 |     unimplemented!();
  |     ^^^^^^^^^^^^^
  |
  = note: import resolution is stuck, try simplifying macro imports

error: aborting due to previous error

error: Could not compile `repro`.

To learn more, run the command again with --verbose.

@djrenren @petrochenkov

[petrochenkov]

This is more of a consequence of #52234 than #[test] infra changes.
It caused pretty bad fallout in general, but it should be fixable, I hope to send a PR in a few days.