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

@@ -37,16 +37,11 @@ describe('Application life cycle test', function () {
     }
 
     async function loginGiteaOIDC() {
-        await clearCache();
         await goto(`https://${app.fqdn}/user/login`, /Sign in with/);
         await click(/Sign in with/);
         await loginOIDC('Milestones');
     }
 
-    async function logout() {
-        await clearCache();
-    }
-
     async function addPublicKey() {
         const keyPath = path.join(import.meta.dirname, 'id_ed25519');
         fs.chmodSync(keyPath, 0o600);
@@ -93,7 +88,7 @@ describe('Application life cycle test', function () {
     });
 
     it('can admin login', adminLogin);
-    it('can logout', logout);
+    it('can logout', clearCache);
 
     it('can login', loginGiteaOIDC);
 
@@ -134,7 +129,7 @@ describe('Application life cycle test', function () {
     });
 
     it('can admin login (no sso)', adminLogin);
-    it('can logout', logout);
+    it('can logout', clearCache);
 
     it('uninstall app (no sso)', cloudronCli.uninstall);
 
@@ -151,7 +146,7 @@ describe('Application life cycle test', function () {
     it('can update', cloudronCli.update);
 
     it('can admin login', adminLogin);
-    it('can logout', logout);
+    it('can logout', clearCache);
 
     it('can login', loginGiteaOIDC);
     it('can clone the url', cloneRepo);