vdoc: fix flaky location of output path in vdoc_file_test.v (#27020)

Author: kimshrier

cmd/tools/vdoc/vdoc_file_test.v

@@ -72,6 +72,9 @@ fn test_out_path() {
 	test_path := os.join_path(os.vtmp_dir(), 'vdoc_test_${rand.ulid()}')
 	test_mod_path := os.join_path(test_path, small_pure_v_vlib_module)
 	os.mkdir_all(test_path)!
+	// Sentinel v.mod so that vdoc's get_parent_mod() stops climbing here instead of
+	// picking up stray .v files in ancestor tmp dirs (e.g. /private/tmp on macOS).
+	os.write_file(os.join_path(test_path, 'v.mod'), "Module { name: 'vdoc_test' }\n")!
 	defer {
 		os.chdir(vroot) or {}
 		os.rmdir_all(test_path) or {}
@@ -82,7 +85,7 @@ fn test_out_path() {
 
 	// Relative input with default output path.
 	os.execute_opt('${vexe} doc -f html -m ${small_pure_v_vlib_module}')!
-	output_path := os.join_path(mod_path, '_docs', '${small_pure_v_vlib_module}.html')
+	output_path := os.join_path(test_mod_path, '_docs', '${small_pure_v_vlib_module}.html')
 	assert os.exists(output_path), output_path
 
 	// Custom out path (no `_docs` subdir).