test_etc.rb: omit unrunnable tests

* test/etc/test_etc.rb (test_sysconf, test_confstr, test_pathconf):
  define test cases only if corresponding constants are available.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46009 b2dd03c8-39d4-4d8f-98ff-823fe69b080e