Explicitly define @default_specifications_dir for tests

Resetting `@default_specifications_dir` to nil means that the
`Gem.default_specifications_dir` needs to be invoked. However, given
that this method might be overriden by operating_system.rb and similar,
this might lead to various test failures. Providing the default value
makes the issues go away.

test/rubygems/helper.rb

@@ -372,7 +372,7 @@ def setup
 
     ENV["GEM_PRIVATE_KEY_PASSPHRASE"] = PRIVATE_KEY_PASSPHRASE
 
-    Gem.instance_variable_set(:@default_specifications_dir, nil)
+    Gem.instance_variable_set(:@default_specifications_dir, File.join(@gemhome, "specifications", "default"))
     if Gem.java_platform?
       @orig_default_gem_home = RbConfig::CONFIG["default_gem_home"]
       RbConfig::CONFIG["default_gem_home"] = @gemhome