Fix some test failures in compiletest branch

[felix91gr]

Helping @oli-obk with #3905

[mati865]

CI errors:

error: identical conversion
  --> tests/compile-test.rs:80:16
   |
80 |     for dir in fs::read_dir(&config.src_base)? {
   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `fs::read_dir(&config.src_base)?()`: `fs::read_dir(&config.src_base)?`
   |
   = note: `-D clippy::identity-conversion` implied by `-D clippy::all`
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#identity_conversion
error: identical conversion
  --> tests/compile-test.rs:83:21
   |
83 |         for file in fs::read_dir(&dir_path)? {
   |                     ^^^^^^^^^^^^^^^^^^^^^^^^ help: consider removing `fs::read_dir(&dir_path)?()`: `fs::read_dir(&dir_path)?`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#identity_conversion
error: aborting due to 2 previous errors

[felix91gr]

I know. I couldn't get rid of those: there seems to be a regression on rustc (or possibly Clippy) that causes that lint to misbehave. See Oliver's comment here