PTY.open is not guaranteed to work. On my machine opening a
pty is prohibited via process control group. On those cases
exceptions shall occur, and that doesn't mean our fault.
Skip those tests on such situations.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@29281 b2dd03c8-39d4-4d8f-98ff-823fe69b080e