Self-closing tags are reported by newer versions of the htmllint
Grunt plugin. They also don't make sense in our HTML files
since they are not XHTML-compliant and they run in HTML mode
anyway.
Ref gh-2157
Creating the label broke while doing a refactoring, the lack of tests
allowed that to go unnoticed. Shouldn't happen again with the extra
tests, covering both controlgroupLabel and spinner.