PHPUnit tests with @runInSeparateProcess fail with PHP 8.0 and Windows

### Reproduction Steps

The LifterLMS unit tests that use the `@runInSeparateProcess` annotation return an error and are not tested when run with PHP 8.0 and Windows 10.

The tests are successful if run with PHP 7.2 or the `@runInSeparateProcess` annotation is removed.

### Expected Behavior

The tests pass.

### Actual Behavior

The tests fail with a "The filename, directory name, or volume label syntax is incorrect." message.

### Error Messages / Logs

"The filename, directory name, or volume label syntax is incorrect."

This issue has been recreated:
+ [X] Locally
+ [ ] On a staging site
+ [ ] On a production website
+ [ ] With only LifterLMS and a default theme