Add basic test for HTML report generation

[denny]

This is a naive copy-and-adapt of the existing json_report_test.rb; I created it to see if I could work up a test for #372, but I think I'm going to have to concede defeat on that - there's too much mocking going on that I don't understand properly - but I thought this file might at least be a starting point for somebody else to use as a base for adding that test in the future?

Let me know if this looks useful, and if so I'll squash it and do the CHANGELOG thing - probably after you do your imminent release.

[denny]

Seems to be failing on Ruby 2.5 and 2.6 (but not 2.4, wut??) on Travis CI - again, at first glance I don't think I know what's going on there, sorry.

Least useful PR ever 😄

[denny]

Oh, except... I just ran the test suite under 2.7.2 on my machine and it failed with the same error the other two have on Travis. Then I blew away all the things that are ignored in the .gitignore - tmp, the output in test/samples, etc etc - and the tests ran successfully.

Then I switched to 2.6.6 and the tests ran okay without clearing temp files again (?!)

Then 2.5.8, where they had the same error, which again cleared when I cleared the temp files.

I still have no idea what's going on, but maybe somebody else can guess based on that lot.

[denny]

In vaguely related news, the tests fail early for me on Ruby 3.0.0, with "File does not exist: rexml/document"

Presumably related to this in 3.0.0 release notes:

The following default gems are now bundled gems.

• rexml

I'm not sure what adding it to the Gemfile would do to older Rubies, so I haven't.

[nunosilva800]

Thanks for this investigation @denny I'll have a better look towards the weekend. Regarding ruby3, we should add that to the travis config, and allow failures for now. Then create issues to address those failures and prepare a release that supports ruby3. :)

[etagwerker]

Continuing this work over here: https://github.com/whitesmith/rubycritic/pull/483