test_inadvertent_creation.rb: split test

* test/-ext-/symbol/test_inadvertent_creation.rb: split
  test_module_const_defined? from test_module_const_get, and
  assertion for the latter.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48473 b2dd03c8-39d4-4d8f-98ff-823fe69b080e