testing: introduce test only extensions

As it is now, there are filters in the repo that are supposed to be used in tests only ([example](https://github.com/envoyproxy/envoy-mobile/blob/main/library/common/extensions/filters/http/test_accessor/filter.h)) but are compiled as any other production extensions.

Add a way for us to define test Envoy extensions for objects that are supposed to be used in tests only.

Test extensions should:
* not be built into the core library
* live in a separate directory
* would be registered separately from "normal" extensions

See this [GH comment](https://github.com/envoyproxy/envoy-mobile/pull/1602#discussion_r673481022) for more context.