Explicitly set the Charset to use, and look for the usual format.

In the GitLab CI environment, the command:

@header("Content-Type: text/plain");

Generates the Content-Type line of:

Content-Type: text/plain; charset=UTF-8

But on my workstation it generates this:

Content-Type: text/plain;charset=UTF-8

By adding that charset to our call to @header, we receive a
predictable result.

testing/test-RRULE-v2.php

@@ -12,7 +12,7 @@ $c->dbg = array();
 require_once("RRule.php");
 require_once('AwlQuery.php');
 
-@header("Content-Type: text/plain");
+@header("Content-Type: text/plain; charset=UTF-8");
 
 echo <<<EOTXT
 Testing the RRule v2 Library

testing/tests/regression-suite/0831-Spec-RRULE-1.result

@@ -3,7 +3,7 @@ Date: Dow, 01 Jan 2000 00:00:00 GMT
 DAV: 1, 2, 3, access-control, calendar-access, calendar-schedule
 DAV: extended-mkcol, bind, addressbook, calendar-auto-schedule, calendar-proxy
 Content-Length: 7623
-Content-Type: text/plain;charset=UTF-8
+Content-Type: text/plain; charset=UTF-8
 
 #!/usr/bin/php
 Testing the RRule v2 Library