chore(rln-relay): Laundry list for tests

[rymnc]

Background

There are a few instances of code in

• test_waku_rln_relay.nim
• test_waku_rln_relay_onchain.nim
  that need to be refactored according to the Status and/or nwaku style guide.

Details

Address the issues and adhere to the style guides.

Acceptance criteria

• Replace var with let
• Move checks to the end of the test, or replace with require
• Use parentheses for isOk, isErr and associated procs

cc: @LNSD

[rymnc]

Completed in #1452