Fix doctests on CI

Failure in doctests: https://gitlab.com/igrep/main-tester/-/jobs/61213082
Success after disabling doctests: https://gitlab.com/igrep/main-tester/pipelines/20057764

### Failure in src/Test/Main.hs:118: expression `withStdin "abcde" main'
expected: edcba
 but got: *** Exception: <stdin>: hDuplicate: illegal operation (cannot flush the read buffer: underlying device is not seekable)