Merge pull request #5274 from shuujii/fix-the-condition-to-remove-mrbtest-when-rake-clean

Fix the condition to remove `mrbtest` when `rake clean`

tasks/test.rake

@@ -60,5 +60,6 @@ MRuby.each_target do |build|
 end
 
 task :clean do
-  rm_f "#{MRuby::Build.install_dir}/mrbtest"
+  host = MRuby.targets["host"]
+  rm_f host.exefile("#{host.class.install_dir}/mrbtest") if host && host.test_enabled?
 end