Skip running a DRb test on MinGW

	It hangs even after a retry
	https://github.com/ruby/ruby/runs/7966439530?check_suite_focus=true

	We contacted GitHub Suppport about this before, and we concluded that
	the problem is on our end. Unfortunately we don't have a bandwidth to
	fix this MinGW problem, so until we get to work on it, this should be
	just skipped to avoid a sporadic CI timeout.
	---
	 test/drb/drbtest.rb      | 1 +
	 test/drb/test_drbssl.rb  | 4 ++++
	 test/rinda/test_rinda.rb | 2 +-
	 3 files changed, 6 insertions(+), 1 deletion(-)