`early_warn` is called
Skip `colored-session-opt-error.rs` on Windows hosts
This is very cursed as to why it fails on Windows CI specifically:
- The test emits a *warning*.
- *Warnings*, and only warnings *specifically*, have a different
256-color between Windows and non-Windows hosts (other levels
`set_intense(true)` unconditionally):
e69c7306e2/compiler/rustc_errors/src/lib.rs (L1792-L1794)
Therefore, I added `// ignore-windows` test header to skip this test on
Windows (it's sufficient to test color is enabled on at least one
non-Windows host).
