Fix race condition in t/ui/18-tests-details.t

* The synchronization via `wait_for_ajax_and_animations` did likely not
  work because there is probably still asynchronicity going on despite the
  attempt to disable animations explicitly. Note that the relevant Ajax
  code still uses jQuery so this is *not* a regression of 87a7e36.
* Use `wait_for_element` like in many other cases.
* Avoid running into `Can't call method "is_enabled" on an undefined value`
  in case the test fails.
* Remove code that attempted to find more than one element by ID. (Even if
  the ID was not unique the test would not be able to find this violation.)
* See https://progress.opensuse.org/issues/167656

t/ui/18-tests-details.t

@@ -77,13 +77,12 @@ sub current_tab { $driver->find_element('.nav.nav-tabs .active')->get_text }
 # returns the contents of the candidates combo box as hash (key: tag, value: array of needle names)
 sub find_candidate_needles {
     # ensure the candidates menu is visible
-    my @candidates_menus = $driver->find_elements('#candidatesMenu');
-    is(scalar @candidates_menus, 1, 'exactly one candidates menu present at a time');
+    my $candidates_menu = wait_for_element(selector => '#candidatesMenu', is_displayed => 1) or return {};
     # save implicit waiting time as long as we are only looking for elements
     # that should be visible already
     disable_timeout;
-    return {} unless $candidates_menus[0]->is_enabled;
-    $candidates_menus[0]->click();
+    return {} unless $candidates_menu->is_enabled;
+    $candidates_menu->click;
 
     # read the tags/needles from the HTML structure
     my @section_elements = $driver->find_elements('#needlediff_selector ul table');
@@ -152,7 +151,6 @@ subtest 'show job modules execution time' => sub {
 
 subtest 'displaying image result with candidates' => sub {
     $driver->find_element('[href="#step/bootloader/1"]')->click();
-    wait_for_ajax;
     my $needles = find_candidate_needles;
     is_deeply($needles, {'inst-bootmenu' => []}, 'correct tags displayed') or diag explain $needles;
 };
@@ -611,7 +609,6 @@ sub test_with_error {
     # check whether candidates are displayed as expected
     my $random_number = int(rand(100000));
     $driver->get("/tests/99946?prevent_caching=$random_number#step/yast2_lan/1");
-    wait_for_ajax_and_animations;
     is_deeply(find_candidate_needles, $expect, $test_name // 'candidates displayed as expected');
 }