Run tests with --disable-gems.

This makes the test suite execute with `--disable-gems` option. This should
help to reduce the initial footprint of loaded libraries and reduce the risk
of inadvertently testing system RubyGems instead of upstream version.

.github/workflows/ubuntu-rubygems.yml

@@ -34,7 +34,7 @@ jobs:
       - name: Install Dependencies
         run: rake setup
       - name: Run Test
-        run: rake test
+        run: RUBYOPT='--disable-gems -w  -Ilib:test:bundler/lib' ruby -e 'Dir.glob "./test/**/test_*.rb", &method(:require)'
         if: "!startsWith(matrix.ruby.name, 'truffleruby') && !startsWith(matrix.ruby.name, 'jruby')"
       - name: Run Test (JRuby)
         run: JRUBY_OPTS=--debug rake test