Fixed: Form cache test not properly ported from Drupal

[jenlampton]

This is a follow up to Issue #3375: Make cache_form expiration configurable, to mitigate runaway cache_form tables.

By @klonos and @herbdool.

FormsFormCacheTestCase was added to the codebase, but never adapted from Drupal. It extends DrupalWebTestCase instead of BackdropWebTestCase and still has a getInfo() method. It was never added into simpletest.tests.info so has likely never been run.

I stumbled across this while working on another issue that adds a test to this file. When I did a find/replace for DrupalWebTestCase/BackdropWebTestCase I caught an extra one I wasn't expecting!

[indigoxela]

A PR is ready for review.

Note that Backdrop does form caching via tempstore, so cache_clear_all() would not affect it. The test now does it the same way as a cron run would do it.

[jenlampton]

PR looks great, thanks for the quick turn-around @indigoxela !! RTBC :D

[klonos]

...but never adapted from Drupal.

Doh! ...my bad 😓

[quicksketch]

Thanks @indigoxela and @jenlampton! And no problem @klonos! We missed it in review too. Merged backdrop/backdrop#3796 into 1.x and 1.20.x.