[rubygems/rubygems] Fix tests checking intermediate files

Some tests check that the shared objects are actually installed, but
checking an intermediate build file instead of the installed one.

https://github.com/rubygems/rubygems/commit/ad526073b0

test/rubygems/test_gem_installer.rb

@@ -1559,7 +1559,7 @@ gem 'other', version
         write_file File.join(@tempdir, file)
       end
 
-      so = File.join(@spec.gem_dir, "#{@spec.name}.#{RbConfig::CONFIG["DLEXT"]}")
+      so = File.join(@spec.extension_dir, "#{@spec.name}.#{RbConfig::CONFIG["DLEXT"]}")
       assert_path_not_exist so
       use_ui @ui do
         path = Gem::Package.build @spec

test/rubygems/test_require.rb

@@ -691,7 +691,7 @@ class TestGemRequire < Gem::TestCase
 
     spec.files += ["extconf.rb", "depend", "#{name}.c"]
 
-    so = File.join(spec.gem_dir, "#{name}.#{RbConfig::CONFIG["DLEXT"]}")
+    so = File.join(spec.extension_dir, "#{name}.#{RbConfig::CONFIG["DLEXT"]}")
     assert_path_not_exist so
 
     path = Gem::Package.build spec