MDL-67673 phpunit: Remove expectedException annotations

While this is not strictly required, because removal will
happen in PHPUnit 9.0, we are already getting rid of all
uses in core.

From release notes:https://phpunit.de/announcements/phpunit-8.html

The annotations `@expectedException`, `@expectedExceptionCode`,
`@expectedExceptionMessage`, and `@expectedExceptionMessageRegExp`
are now deprecated.
Using these annotations will trigger a deprecation warning
in PHPUnit 8 and in PHPUnit 9 these annotations will be removed.

Also, all uses of expectExceptionMessageRegExp() has been moved
to expectExceptionMessageMatches(). See https://github.com/sebastianbergmann/phpunit/issues/3957

TODO: Various weirdness found while doing the changes with these tests:
- vendor/bin/phpunit lib/tests/exporter_test.php (created MDL-69700)
- vendor/bin/phpunit competency/tests/external_test.php (same issue than prev one)
- vendor/bin/phpunit question/engine/tests/questionengine_test.php (created MDL-69624)
- vendor/bin/phpunit lib/tests/event_test.php (created MDL-69688)

question/engine/tests/questionattemptiterator_test.php

@@ -87,10 +87,8 @@ class question_attempt_iterator_test extends advanced_testcase {
         $this->assertFalse(isset($this->iterator[3]));
     }
 
-    /**
-     * @expectedException moodle_exception
-     */
     public function test_offsetGet_before_start() {
+        $this->expectException(moodle_exception::class);
         $step = $this->iterator[0];
     }
 
@@ -102,24 +100,18 @@ class question_attempt_iterator_test extends advanced_testcase {
         $this->assertSame($this->qas[2], $this->iterator[2]);
     }
 
-    /**
-     * @expectedException moodle_exception
-     */
     public function test_offsetGet_past_end() {
+        $this->expectException(moodle_exception::class);
         $step = $this->iterator[3];
     }
 
-    /**
-     * @expectedException moodle_exception
-     */
     public function test_cannot_set() {
+        $this->expectException(moodle_exception::class);
         $this->iterator[0] = null;
     }
 
-    /**
-     * @expectedException moodle_exception
-     */
     public function test_cannot_unset() {
+        $this->expectException(moodle_exception::class);
         unset($this->iterator[2]);
     }
 }