MDL-81266 phpunit: Remove expect(Deprecation|Notice|Warning|Error)

PHPUnit 9.6 has deprecated all the expect(Deprecation|Notice|Warning|Error)
assertions, so we have to move away from them.

In core, we only had 2 cases, one easily fixed by getting rid of it,
because, for php >= 80 it's an assert-able exception.

And the other replaced with code that, temporarily, sets a custom
error handler that converts any specified E_ to an asset-able
exception.

Note that tests playing with error handlers should, always, be
run in separate process, to avoid problems or conflicts with
PHPUnit / Moodle own error handlers.

lib/phpunit/tests/advanced_test.php

@@ -369,7 +369,15 @@ class advanced_test extends \advanced_testcase {
         ]);
 
         if ($phpwarn) {
-            $this->expectWarning();
+            // Let's convert the warnings into an assert-able exception.
+            set_error_handler(
+                static function ($errno, $errstr) {
+                    restore_error_handler();
+                    throw new \Exception($errstr, $errno);
+                },
+                E_WARNING // Or any other specific E_ that we want to assert.
+            );
+            $this->expectException(\Exception::class);
         }
         $this->assertEventContextNotUsed($event);
     }