test_weakmap.rb: add

* test/ruby/test_weakmap.rb: test for ObjectSpace::WeakMap.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44092 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
gc.c: ObjectSpace::WeakMap#size

* gc.c (wmap_size): add ObjectSpace::WeakMap#size and #length.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44093 b2dd03c8-39d4-4d8f-98ff-823fe69b080e