Ignore vendored libraries by rubygems from code coverage

tool/test-coverage.rb

@@ -7,6 +7,9 @@ TEST_COVERAGE_DATA_FILE = "test-coverage.dat"
 FILTER_PATHS = %w[
   tool/lib
   lib/bundler/vendor
+  lib/rubygems/resolver/molinillo
+  lib/rubygems/tsort
+  lib/rubygems/optparse
   test
 ]