Suggest library/std when running all stage 0 tests

Author: jyn514

src/bootstrap/test.rs

@@ -970,7 +970,8 @@ impl Step for Compiletest {
         if builder.top_stage == 0 && env::var("COMPILETEST_FORCE_STAGE0").is_err() {
             eprintln!("\
 error: `--stage 0` runs compiletest on the beta compiler, not your local changes, and will almost always cause tests to fail
-help: use `--stage 1` instead
+help: to test the compiler, use `--stage 1` instead
+help: to test the standard library, use `--stage 0 library/std` instead
 note: if you're sure you want to do this, please open an issue as to why. In the meantime, you can override this with `COMPILETEST_FORCE_STAGE0=1`."
             );
             std::process::exit(1);