I've found that with r39919 and r39925 TestMethod#test_bound_method_entry wait child process forever because assert_ruby_status call invoke_ruby with capture_stdout=false. I'll add r39905 too.
This issue was solved with changeset r40254.
Narihiro, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.
* test/ruby/envutil.rb (assert_ruby_status): include stderr output. unlinked method entry. [Bug #8100] [ruby-core:53439]
* proc.c (bm_free): need to clean up the mark flag of a free and