refactor: Lift out log-out call from individual tests to common.ts.

We recently added the commit to add the log-out call
after each test (52706908b).

This commit cleans that approach by using
just one log-out call after the test is executed at
`common.ts`

frontend_tests/puppeteer_lib/common.ts

@@ -581,6 +581,7 @@ class CommonUtils {
 
         try {
             await test_function(page);
+            await this.log_out(page);
 
             if (page_errored) {
                 throw new Error("Page threw an error");