Fixed tests from streaming changes

- Added testing check to buffer stop/clear on streaming output due to
  interference during tests.
- Made content-disposition header a little safer in download responses.
- Also aligned how we check for testing environment.

resources/views/common/export-styles.blade.php

@@ -1,5 +1,5 @@
 <style>
-    @if (!app()->environment('testing'))
+    @if (!app()->runningUnitTests())
         {!! file_get_contents(public_path('/dist/export-styles.css')) !!}
     @endif
 </style>