MDL-40552 course: fixup unit test

Previously there was an assertion which was never used, now
we ensure to test the course themes functionality each time.

course/tests/externallib_test.php

@@ -335,6 +335,8 @@ class core_course_externallib_testcase extends externallib_advanced_testcase {
 
         // Enable course completion.
         set_config('enablecompletion', 1);
+        // Enable course themes.
+        set_config('allowcoursethemes', 1);
 
         // Set the required capabilities by the external function
         $contextid = context_system::instance()->id;
@@ -415,10 +417,7 @@ class core_course_externallib_testcase extends externallib_advanced_testcase {
                 $this->assertEquals($courseinfo->defaultgroupingid, $course2['defaultgroupingid']);
                 $this->assertEquals($courseinfo->completionnotify, $course2['completionnotify']);
                 $this->assertEquals($courseinfo->lang, $course2['lang']);
-
-                if (!empty($CFG->allowcoursethemes)) {
-                    $this->assertEquals($courseinfo->theme, $course2['forcetheme']);
-                }
+                $this->assertEquals($courseinfo->theme, $course2['forcetheme']);
 
                 // We enabled completion at the beginning of the test.
                 $this->assertEquals($courseinfo->enablecompletion, $course2['enablecompletion']);