Driver cache has no limit

[Rinzwind]

There’s no limit to the number of BPRemoteWebDriver instances that BPWebDriverManagerResource caches, which can cause performance degradation even with a low number of cached instances when using a CI setup with constrained resources. There should either be a configurable limit, or support for caching multiple instances should just be dropped.

Note that on Pharo, prior to smalltalkCI pull request #662, test resources were not shared between test classes.