test(cli): lock in trailing newline on gitignored-tracked warning

Guards against a silent refactor from Fprintln to Fprint dropping the
terminating newline that downstream output relies on.

cmd/ninja/cmd/plugin_test.go

@@ -203,6 +203,9 @@ func TestGitignoredTrackedWarning_FiresWhenTrackedFileMatchesGitignore(t *testin
 	if !strings.Contains(out, "git rm --cached") {
 		t.Errorf("warning should suggest `git rm --cached`, got: %q", out)
 	}
+	if !strings.HasSuffix(out, "\n") {
+		t.Errorf("warning should end with a newline (Fprintln), got: %q", out)
+	}
 }
 
 func TestGitignoredTrackedWarning_NoopWhenNothingMatches(t *testing.T) {