Propagating MAKEFLAGS to children running test/unit caused stuck
tests with GNU make when "-jN" is passed in both the make(1)
command-line and the "TESTS=" variable; because the forked child
process would see MAKEFLAGS and try to use jobserver on its own.

This is regression caused by r64399
(commit b53fadfd,
"process.c: defaults to close_others false");
but that change also fixed a regression when close-on-exec
became the default in 2.0 :<

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66404 b2dd03c8-39d4-4d8f-98ff-823fe69b080e