[Test] Add test for csv detection

test/functional/cases/001_merged/350_magic.robot

   ...  MAGIC_SYM_BAT_54
   ...  MAGIC_SYM_ICS_55
   ...  MAGIC_SYM_VCF_56
+  ...  MAGIC_SYM_CSV_57

test/functional/messages/gargantua.eml

 hg0KTjo70JDQstGC0L7QutC+0YDQtdC10YY7OzsNClRFTDtUWVBFPUNFTEw6MjYy
 LTc3Mg0KVEVMO1RZUEU9SE9NRTorNzkwMDUyNjI3NzINCkVORDpWQ0FSRA0K
 
+--XXX
+Content-Type: text/csv
+Content-Transfer-Encoding: base64
+X-Real-Type: csv
+
+ImZvbyIsImJhciIsImJheiIKIjEiLCIyIiwiMyIK
 
---XXX--
+--XXX--