rubygems/test_case.rb: fix errors in mkmf tests

* lib/rubygems/test_case.rb (Gem#teardown): BASERUBY is not set
  usually.  fix errors in mkmf tests.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44526 b2dd03c8-39d4-4d8f-98ff-823fe69b080e