fails due to windows path separators

[robjtede]

See here for CI failures:

actix/actix-net#133
https://github.com/actix/actix-net/pull/133/checks?check_run_id=620179757

Relevant Lines:

EXPECTED:

| |_^ consider adding a `main` function to `$DIR/tests/trybuild/main-02-only-async.rs`

ACTUAL OUTPUT:

| |_^ consider adding a `main` function to `$DIR\tests/trybuild/main-02-only-async.rs`
                                           ^^^^^^^

Read some things in other closed issues about normalization but this seems distinct.