fix concurrent test.

	* test/rubygems/test_require.rb (test_concurrent_require):
	  Synchronizations should be in ensure clause. Sometimes
	  `require` fails (not sure why) and latch is not released.
	  Such case introduces unlimited awaiting.
	  This patch soleve this problem.


	skip some tests so that no failure occurs in root privilege

	Some tests had failed on `sudo make test-all`, mainly because root can
	access any files regardless of permission.  This change adds `skip`
	guards into such tests.

	Note that almost all tests in which `skip` guards is added, already have
	"windows" guard.  This is because there is no support to avoid read
	access by owner on Windows.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@62834 b2dd03c8-39d4-4d8f-98ff-823fe69b080e