Add ability to have unit tests for ROCKSDB_PLUGINS (#11052)

Add ability to have unit tests for ROCKSDB_PLUGINS (#11052)

Summary:
This is based on speedb PR [143](https://github.com/speedb-io/speedb/pull/143).

This PR adds the ability to add a xxx_TESTS variable to the make or cmake files for a plugin.  When set, those files will be added to the unit tests built and executed by the corresponding make system.

Note that the rule for building plugin tests via make could be expanded to almost every other unit test in RocksDB.  This expansion would allow for a much smaller/simpler Makefile and make it easier to add new test files to RocksDB.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11052

Reviewed By: cbi42

Differential Revision: D42212269

Pulled By: ajkr

fbshipit-source-id: d02668f7f4466900d63c90bb4f7962d23fcc7114