gprs: Implement PURGE_MS GSUP messages

This commit implements the encoding and decoding of the messages

  - Purge MS Request
  - Purge MS Error
  - Purge MS Result

and adds corresponding tests.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/tests/gprs/gprs_test.ok b/openbsc/tests/gprs/gprs_test.ok
index eda1f36..4825b67 100644
--- a/openbsc/tests/gprs/gprs_test.ok
+++ b/openbsc/tests/gprs/gprs_test.ok
@@ -24,4 +24,7 @@
   Testing Location Cancellation Request
   Testing Location Cancellation Error
   Testing Location Cancellation Result
+  Testing Purge MS Request
+  Testing Purge MS Error
+  Testing Purge MS Result
 Done.