MDL-81522 phpunit: Add missing void return type to all tests

While this change is not 100% required now, it's good habit
and we are checking for it since Moodle 4.4.

All the changes in this commit have been applied automatically
using the moodle.PHPUnit.TestReturnType sniff and are, exclusively
adding the ": void" return types when missing.

lib/tests/rsslib_test.php

@@ -45,7 +45,7 @@ class rsslib_test extends \advanced_testcase {
         \moodle_simplepie::reset_cache();
     }
 
-    public function test_getfeed() {
+    public function test_getfeed(): void {
         $feed = new \moodle_simplepie($this->getExternalTestFileUrl('/rsstest.xml'), self::TIMEOUT);
 
         $this->assertInstanceOf('\moodle_simplepie', $feed);
@@ -99,7 +99,7 @@ EOD;
     /*
      * Test retrieving a url which doesn't exist.
      */
-    public function test_failurl() {
+    public function test_failurl(): void {
         global $CFG;
 
         // We do not want this in php error log.
@@ -113,7 +113,7 @@ EOD;
     /*
      * Test retrieving a url with broken proxy configuration.
      */
-    public function test_failproxy() {
+    public function test_failproxy(): void {
         global $CFG;
 
         $oldproxy = $CFG->proxyhost;
@@ -133,7 +133,7 @@ EOD;
     /*
      * Test retrieving a url which sends a redirect to another valid feed.
      */
-    public function test_redirect() {
+    public function test_redirect(): void {
         $feed = new \moodle_simplepie($this->getExternalTestFileUrl('/rss_redir.php'), self::TIMEOUT);
 
         $this->assertNull($feed->error());
@@ -146,7 +146,7 @@ EOD;
      *
      * @covers ::rss_get_userid_from_token
      */
-    public function test_rss_get_userid_from_token() {
+    public function test_rss_get_userid_from_token(): void {
         global $USER;
 
         $this->resetAfterTest();