crypt: Test for the NSE race bug #3110

Adds a test for #3110
that fails before the fix and passes afterwards.

Author: andybalaam

testing/matrix-sdk-integration-testing/src/tests.rs

@@ -1,6 +1,7 @@
 mod auth;
 mod e2ee;
 mod invitations;
+mod nse;
 mod reactions;
 mod redaction;
 mod repeated_join;