Add clarunit bullet in fuzz-testing README

Author: BowTiedRadone

guides-and-tutorials/testing-smart-contracts/README.md

@@ -7,5 +7,6 @@ This section covers testing Clarity contracts.
 
 * [Fuzz Testing](./fuzz-testing.md): Use Rendezvous to hammer your contract with random
   inputs. It helps expose edge cases and vulnerabilities.
+* [Clarity Unit Testing](https://github.com/stacks-network/clarunit)
 
 More guides will follow.