Replace code-assert with alternative checkstyle test

[dbwiddis]

I also think we should get rid of guru.nidi:code-assert alltogether as it has a humongous dependency tree and it's only used in one testcase:

Originally posted by @mprins in #1796 (comment)

The one testcase code-assert is used in is completely intended to give a runtime test enforcement of checkstyle issues. Prior to using this, we were only finding these issues in the Appveyor builds, and when I say "finding" I mean failing builds and spending 10 minutes poring over the log to try to figure out the issue, instead of having it print out nicely.

I'm okay with removing code-assert if we properly configure the checkstyle plugin to fail the build on warnings, see here, for example. Other potential options are using one of our existing CI integrations that integrates with PRs, or a Github Action that tests checkstyle, like this one