Fix flaky spec failure

Dirty tests should clean up after themselves, not cleanup other dirty specs
leaks.

In this case, the specs were cleaning up dirty state before them, so
they could work, but leaking state after being running.

spec/result_merger_spec.rb

@@ -5,7 +5,7 @@
 require "timeout"
 
 describe SimpleCov::ResultMerger do
-  before do
+  after do
     SimpleCov::ResultMerger.clear_resultset
     File.delete(SimpleCov::ResultMerger.resultset_path) if File.exist?(SimpleCov::ResultMerger.resultset_path)
   end