ignore tests which raised LoadError.
* test/drb/drbtest.rb, test/ruby/test_beginendblock.rb,
test/ruby/test_system.rb: avoid requiring same file twice.
* test/drb/test_drbssl.rb, test/drb/test_drbunix.rb: should not use
ARGV unless invoked directly. do not create test cases unless
required libraries are available.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@4830 b2dd03c8-39d4-4d8f-98ff-823fe69b080e