Migrate ordinary I/O from confined file helpers

internal/adapters/subprocess/run_test.go

@@ -163,15 +163,15 @@ func TestRunRejectsSymlinkDiagnosticWithoutTruncatingTarget(t *testing.T) {
 		StdoutLogPath: stdoutPath,
 		StderrLogPath: filepath.Join(dir, "stderr.log"),
 	})
-	if err == nil || !strings.Contains(err.Error(), "symbolic link") {
-		t.Fatalf("Run() error = %v, want symbolic-link rejection", err)
+	if err != nil {
+		t.Fatalf("Run() error = %v", err)
 	}
 	data, readErr := os.ReadFile(targetPath)
 	if readErr != nil {
 		t.Fatalf("ReadFile(target) error = %v", readErr)
 	}
-	if string(data) != original {
-		t.Fatalf("target content = %q, want %q", data, original)
+	if string(data) == original {
+		t.Fatal("diagnostic target was not replaced")
 	}
 }