[rubygems/rubygems] Provide more insightful test error output

Original output:

~~~
Failure: test_realworld_upgraded_default_gem(TestGemRequire): <false> is not true.
/mnt/test/rubygems/test_require.rb:474:in `test_realworld_upgraded_default_gem'
     471:     File.write(path, code)
     472:
     473:     output = Gem::Util.popen({ "GEM_HOME" => @gemhome }, *ruby_with_rubygems_in_load_path, path).strip
  => 474:     assert $?.success?
     475:     refute_empty output
     476:     assert_equal "999.99.9", output.lines[0].chomp
     477:     # Make sure only files from the newer json gem are loaded, and no files from the default json gem
~~~

New output:

~~~
Failure: test_realworld_upgraded_default_gem(TestGemRequire)
/mnt/test/rubygems/test_require.rb:475:in `test_realworld_upgraded_default_gem'
     472:
     473:     output = Gem::Util.popen({ "GEM_HOME" => @gemhome }, *ruby_with_rubygems_in_load_path, path).strip
     474:     refute_empty output
  => 475:     assert_equal "999.99.9", output.lines[0].chomp
     476:     # Make sure only files from the newer json gem are loaded, and no files from the default json gem
     477:     assert_equal ["#{@gemhome}/gems/json-999.99.9/lib/json.rb"], output.lines.grep(%r{/gems/json-}).map(&:chomp)
     478:     assert $?.success?
<"999.99.9"> expected but was
<"/mnt/tmp/test_rubygems_20231110-36663-of405r/test_realworld_upgraded_default_gem.rb:3:in `<main>': undefined method `version' for nil:NilClass (NoMethodError)">

diff:
? 999                                                                             .99.9
? /mnt/tmp/test_rubygems_20231110-36663-of405r/test_realworld_upgraded_default_gem rb:3:in `<main>': undefined method `version' for nil:NilClass (NoMethodError)
? ??? ????
~~~

It is more valuable to check the command output then the error code. If
the command fails for some reason, the output probably contains some
detail, while checking the return code tells not much.

rubygems/rubygems@b76062e852

test/rubygems/test_require.rb

@@ -471,11 +471,11 @@ def test_realworld_upgraded_default_gem
     File.write(path, code)
 
     output = Gem::Util.popen({ "GEM_HOME" => @gemhome }, *ruby_with_rubygems_in_load_path, path).strip
-    assert $?.success?
     refute_empty output
     assert_equal "999.99.9", output.lines[0].chomp
     # Make sure only files from the newer json gem are loaded, and no files from the default json gem
     assert_equal ["#{@gemhome}/gems/json-999.99.9/lib/json.rb"], output.lines.grep(%r{/gems/json-}).map(&:chomp)
+    assert $?.success?
   end
 
   def test_default_gem_and_normal_gem