Move testthat from Imports to Suggests

Fixes Note:
Namespace in Imports field not imported from: ‘testthat’
  All declared Imports should be used.

Change-Id: I8795e2facf746f9640d7558203fc944e865f2a64
diff --git a/DESCRIPTION b/DESCRIPTION
index 30530fe..2c2899b 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -45,8 +45,9 @@
     utils,
     httr,
     methods,
-    testthat,
     PTXQC
+Suggests:
+    testthat
 Collate: 
     'KorAPConnection.R'
     'KorAPCorpusStats.R'