MDL-73588 phpunit: Fix assertion to PHPUnit 8.5 one

This issue was backported from 311_STABLE that uses
PHPUnit 9.5, were the regexp assertion was renamed.

This just moves it back to the 8.5 one for 39 and 310 stables.

lib/tests/filelib_test.php

@@ -160,7 +160,7 @@ public function test_download_file_content() {
         $this->assertInstanceOf('stdClass', $response);
         $this->assertSame('200', $response->status);
         $this->assertTrue(is_array($response->headers));
-        $this->assertMatchesRegularExpression('|^HTTP/1\.[01] 200 OK$|', rtrim($response->response_code));
+        $this->assertRegExp('|^HTTP/1\.[01] 200 OK$|', rtrim($response->response_code));
         $this->assertSame('done', $response->results);
         $this->assertSame('', $response->error);