Test reverted to direct call to writeFile.

Abstrated writePersistentRlnCredentials usage causing error,
with readPersistentRlnCredentials

tests/v2/test_waku_rln_relay.nim

@@ -748,7 +748,7 @@ suite "Waku rln relay":
     let path = "testPath.txt"
 
     # Write RLN credentials
-    writePersistentRlnCredentials(path, rlnMembershipCredentials)
+    writeFile(path, pretty(%rlnMembershipCredentials))
 
     var credentials = readPersistentRlnCredentials(path)