[rubygems/rubygems] Extract a `spec` variable

It's consistent with the previous test and makes print debugging easier.

https://github.com/rubygems/rubygems/commit/d7f424df16

test/rubygems/test_gem.rb

@@ -1615,9 +1615,11 @@ class TestGem < Gem::TestCase
 
     Gem.use_paths Gem.dir, [Gem.dir, Gem.user_dir]
 
+    spec = Gem::Dependency.new("m", "1").to_spec
+
     assert_equal \
       File.join(Gem.dir, "gems", "m-1"),
-      Gem::Dependency.new("m","1").to_spec.gem_dir,
+      spec.gem_dir,
       "Wrong spec selected"
   end