[rubygems/rubygems] Use better name for variable

The installed file not always have the `.so` extension.

https://github.com/rubygems/rubygems/commit/6f6681bcb9

Co-authored-by: Eloy Espinaco <eloyesp@gmail.com>

test/rubygems/test_require.rb

@@ -691,13 +691,13 @@ class TestGemRequire < Gem::TestCase
 
     spec.files += ["extconf.rb", "depend", "#{name}.c"]
 
-    so = File.join(spec.extension_dir, "#{name}.#{RbConfig::CONFIG["DLEXT"]}")
-    assert_path_not_exist so
+    extension_file = File.join(spec.extension_dir, "#{name}.#{RbConfig::CONFIG["DLEXT"]}")
+    assert_path_not_exist extension_file
 
     path = Gem::Package.build spec
     installer = Gem::Installer.at path
     installer.install
-    assert_path_exist so
+    assert_path_exist extension_file
 
     spec.gem_dir
   end