`gleam test` fails on windows due to path separator literals in tests

On windows, gleam test fails due to asserts including the path separator:

```
assert test\gleam_panics_test.gleam:104
 test: gleam_panics_test.assert_expression_test
 code: assert e.file == "test/gleam_panics_test.gleam"
 left: "test\\gleam_panics_test.gleam"
right: literal
 info: Assertion failed.
```

And many similar failures, all due to the same issue.