Polish tab count and create button in test cases

- Change button to use confirm variant.
- Update tab count badge to use muted variant for consistency
with other issuable_list apps.

Changelog: changed
EE: true

app/assets/javascripts/issuable_list/components/issuable_tabs.vue

@@ -49,7 +49,7 @@ export default {
           <span :title="tab.titleTooltip">{{ tab.title }}</span>
           <gl-badge
             v-if="tabCounts && isTabCountNumeric(tab)"
-            variant="neutral"
+            variant="muted"
             size="sm"
             class="gl-tab-counter-badge"
           >

ee/app/assets/javascripts/test_case_list/components/test_case_list_root.vue

@@ -341,7 +341,7 @@ export default {
     @sort="handleSortTestCases"
   >
     <template v-if="canCreateTestCase" #nav-actions>
-      <gl-button :href="testCaseNewPath" category="primary" variant="success">{{
+      <gl-button :href="testCaseNewPath" category="primary" variant="confirm">{{
         s__('TestCases|New test case')
       }}</gl-button>
     </template>