Add tests for File.extname

* file.c (rb_file_s_extname): [DOC] add an example.
* test/ruby/test_path.rb (test_extname): add tests.  [Fix GH-978]
  * path starts with dot ('.a.rb')
  * path includes dir name ('a/b/d/test.rb')
  * path includes dir name and dir name starts with dot
    ('.a/b/d/test.rb')

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51359 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

file.c

@@ -4237,6 +4237,7 @@ ruby_enc_find_extname(const char *name, long *len, rb_encoding *enc)
  *
  *     File.extname("test.rb")         #=> ".rb"
  *     File.extname("a/b/d/test.rb")   #=> ".rb"
+ *     File.extname(".a/b/d/test.rb")  #=> ".rb"
  *     File.extname("foo.")	       #=> ""
  *     File.extname("test")            #=> ""
  *     File.extname(".profile")        #=> ""