test_sdbm.rb: only if SDBM

* test/sdbm/test_sdbm.rb (TestSDBM): test only if SDBM is loadable.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41489 b2dd03c8-39d4-4d8f-98ff-823fe69b080e