test/psych is executed without libyaml
ruby 1.9.2dev (2010-03-30 trunk 27106) [x86_64-darwin10.3.0]
2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN
When I make test-all at trunk, test/psych is executed even if the system
does not have libyaml. test/zlib and others aren't executed if building *.so fails.