Fix propagation of test report attributes from killforks

I do not really like the current implementation. But it works for now.

lib/mutant/runner.rb

@@ -124,7 +124,7 @@ module Mutant
       config.isolation.call do
         mutation.insert
         test.run
-      end
+      end.update(test: test, mutation: mutation)
     rescue Isolation::Error
       Result::Test.new(
         test:   test,

spec/unit/mutant/runner_spec.rb

@@ -14,6 +14,10 @@ describe Mutant::Runner do
       super
     end
 
+    def update(attributes)
+      self
+    end
+
     def method_missing(name, *arguments)
       super unless attributes.key?(name)
       fail "Arguments provided for #{name}" if arguments.any?