test: simplify clearCache, drop logout helpers, simplify button selectors

- Drop `await clearCache()` from login/auth helpers; clearing cache before
  every login is no longer necessary now that charlie isolates sessions.
- Replace `logout` helpers with `clearCache` directly in `it()` calls; the
  helpers were either thin wrappers around `clearCache()` or did real
  navigation that is no longer worth the maintenance.
- Simplify `xpath=//button[text()=...]` / `[contains(., ...)]` button text
  selectors to charlie's bare-string (or RegExp for OR-clauses) form.

test/test.js

@@ -32,18 +32,10 @@ describe('Application life cycle test', function () {
     }
 
     async function loginViaOIDC() {
-        await clearCache();
         await goto(`https://${app.fqdn}/i/`);
         await loginOIDC('css=#btn-subscription');
     }
 
-    async function logout() {
-        await goto(`https://${app.fqdn}`, 'css=#stream');
-        await click('css=.dropdown-toggle');
-        await click('xpath=//li/*[contains(@class,"signout")]');
-        await waitFor('css=#loginButton');
-    }
-
     async function addSubscription() {
         const url = 'https://blog.cloudron.io/rss/';
         const addUrl = app.manifest.version === '1.10.0' ? `${baseUrl()}/i/?c=subscription` : `${baseUrl()}/i/?c=subscription&a=add`;
@@ -59,11 +51,11 @@ describe('Application life cycle test', function () {
 
         if (relogin) {
             await sendKeys('css=#passwordPlain', admin_password);
-            await click('xpath=//button[contains(.,"Login")]');
+            await click('Login');
         }
 
         await click('css=#api_enabled');
-        await click('xpath=//button[text()="Submit"]');
+        await click('Submit');
     }
 
     async function checkApiConfiguration() {
@@ -92,7 +84,7 @@ describe('Application life cycle test', function () {
     it('can check configuration', checkApiConfiguration);
     it('subscription exists', subscriptionExists);
     it('can get static extension file', getStaticExtensionFile);
-    it('can logout', logout);
+    it('can logout', clearCache);
     it('uninstall app', cloudronCli.uninstall);
 
     // SSO