headless-browser: Add a --dry-run flag

--dry-run causes headless-browser to collect and then list tests,
without running them.

Userland/Utilities/headless-browser.cpp

@@ -501,9 +501,10 @@ static ErrorOr<void> collect_ref_tests(Vector<Test>& tests, StringView path)
     return {};
 }
 
-static ErrorOr<int> run_tests(HeadlessWebContentView& view, StringView test_root_path, StringView test_glob, bool dump_failed_ref_tests, bool dump_gc_graph)
+static ErrorOr<int> run_tests(HeadlessWebContentView* view, StringView test_root_path, StringView test_glob, bool dump_failed_ref_tests, bool dump_gc_graph, bool dry_run)
 {
-    view.clear_content_filters();
+    if (view)
+        view->clear_content_filters();
 
     TRY(load_test_config(test_root_path));
 
@@ -526,17 +527,26 @@ static ErrorOr<int> run_tests(HeadlessWebContentView& view, StringView test_root
 
     bool is_tty = isatty(STDOUT_FILENO);
 
+    if (dry_run)
+        outln("Found {} tests...", tests.size());
+    else
         outln("Running {} tests...", tests.size());
+
     for (size_t i = 0; i < tests.size(); ++i) {
         auto& test = tests[i];
 
-        if (is_tty) {
+        if (is_tty && !dry_run) {
             // Keep clearing and reusing the same line if stdout is a TTY.
             out("\33[2K\r");
         }
 
         out("{}/{}: {}", i + 1, tests.size(), LexicalPath::relative_path(test.input_path, test_root_path));
 
+        if (dry_run) {
+            outln("");
+            continue;
+        }
+
         if (is_tty)
             fflush(stdout);
         else
@@ -548,7 +558,7 @@ static ErrorOr<int> run_tests(HeadlessWebContentView& view, StringView test_root
             continue;
         }
 
-        test.result = TRY(run_test(view, test.input_path, test.expectation_path, test.mode, dump_failed_ref_tests));
+        test.result = TRY(run_test(*view, test.input_path, test.expectation_path, test.mode, dump_failed_ref_tests));
         switch (*test.result) {
         case TestResult::Pass:
             ++pass_count;
@@ -565,6 +575,9 @@ static ErrorOr<int> run_tests(HeadlessWebContentView& view, StringView test_root
         }
     }
 
+    if (dry_run)
+        return 0;
+
     if (is_tty)
         outln("\33[2K\rDone!");
 
@@ -578,7 +591,7 @@ static ErrorOr<int> run_tests(HeadlessWebContentView& view, StringView test_root
     }
 
     if (dump_gc_graph) {
-        auto path = view.dump_gc_graph();
+        auto path = view->dump_gc_graph();
         if (path.is_error()) {
             warnln("Failed to dump GC graph: {}", path.error());
         } else {
@@ -601,6 +614,7 @@ struct Application : public WebView::Application {
         args_parser.add_option(dump_text, "Dump text and exit", "dump-text", 'T');
         args_parser.add_option(test_root_path, "Run tests in path", "run-tests", 'R', "test-root-path");
         args_parser.add_option(test_glob, "Only run tests matching the given glob", "filter", 'f', "glob");
+        args_parser.add_option(test_dry_run, "List the tests that would be run, without running them", "dry-run");
         args_parser.add_option(dump_failed_ref_tests, "Dump screenshots of failing ref tests", "dump-failed-ref-tests", 'D');
         args_parser.add_option(dump_gc_graph, "Dump GC graph", "dump-gc-graph", 'G');
         args_parser.add_option(resources_folder, "Path of the base resources folder (defaults to /res)", "resources", 'r', "resources-root-path");
@@ -626,6 +640,7 @@ struct Application : public WebView::Application {
     bool is_layout_test_mode { false };
     StringView test_root_path;
     ByteString test_glob;
+    bool test_dry_run { false };
 };
 
 Application::Application(Badge<WebView::Application>, Main::Arguments&)
@@ -646,13 +661,17 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     // FIXME: Allow passing the window size as an argument.
     static constexpr Gfx::IntSize window_size { 800, 600 };
 
-    auto view = TRY(HeadlessWebContentView::create(move(theme), window_size, app->resources_folder));
-
     if (!app->test_root_path.is_empty()) {
+        OwnPtr<HeadlessWebContentView> view;
+        if (!app->test_dry_run)
+            view = TRY(HeadlessWebContentView::create(move(theme), window_size, app->resources_folder));
+
         auto test_glob = ByteString::formatted("*{}*", app->test_glob);
-        return run_tests(*view, app->test_root_path, test_glob, app->dump_failed_ref_tests, app->dump_gc_graph);
+        return run_tests(view, app->test_root_path, test_glob, app->dump_failed_ref_tests, app->dump_gc_graph, app->test_dry_run);
     }
 
+    auto view = TRY(HeadlessWebContentView::create(move(theme), window_size, app->resources_folder));
+
     VERIFY(!WebView::Application::chrome_options().urls.is_empty());
     auto const& url = WebView::Application::chrome_options().urls.first();
     if (!url.is_valid()) {