Fix ErrorReporterAssertions to only subscribe once

Author: byroot

activesupport/lib/active_support/testing/error_reporter_assertions.rb

@@ -42,6 +42,7 @@ def subscribe
 
                 if ActiveSupport.error_reporter
                   ActiveSupport.error_reporter.subscribe(self)
+                  @subscribed = true
                 else
                   raise Minitest::Assertion, "No error reporter is configured"
                 end