Fixes #37163 - Use require_relative for 'coverage_reporter' (theforem…

…an#626)

Replace the usage of require with require_relative for 'coverage_reporter',
ensuring that it is loaded relative to the current file's directory.

lib/minitest/hammer_coverage_plugin.rb

@@ -1,4 +1,4 @@
-require 'coverage_reporter'
+require_relative 'coverage_reporter'
 require "json"
 
 module Minitest