chore: add test for number with underscore parsing

[Krinkle]

What:

Add the test case from https://github.com/less/less.js/issues/2462, as inpired by downstream https://gerrit.wikimedia.org/r/1197310.

Why:

Follows-up https://github.com/less/less.js/pull/2485.

In Less.js 2.6.0, parsing of dimensions changed so that 5_large is seen as one value, instead of as a list containing "5" and "_large".

In maintaining the wikimedia/less.php port, I recently found that we never applied this change because none of the Less.js 3.13 tests covered this behavior.

Checklist:

• [x] Documentation
• [x] Added/updated unit tests
• [x] Code complete

[puckowski]

@Krinkle Thank you for the PR.

I think adding a test for https://github.com/less/less.js/pull/2485 is a good idea.

I'll give some time for other maintainers to weigh in.