benchmark

[RFC] Adding API for setting/getting benchmark_filter flag? (#1254)

* [RFC] Adding API for setting/getting benchmark_filter flag?

This PR is more of a Request-for-comment - open to other ideas/suggestions as well.

Details:
This flag has different implementations(absl vs benchmark) and since the proposal to add absl as a dependency was rejected, it would be nice to have a reliable (and less hacky) way to access this flag internally.
(Actually, reading it isn't much a problem but setting it is).

Internally, we have a sizeable number users to use absl::SetFlags to set this flag. This will not work with benchmark-flags.

Another motivation is that not all users use the command line flag. Some prefer to programmatically set this value.

* fixed build errors

* fix lints again

* per discussion: add additional RunSpecifiedBenchmarks instead.

* add tests

* fix up tests

* clarify comment

* fix stray : in test

* more assertion in test

* add test file to test/CMakeLists.txt

* more test

* make test ISO C++ compliant

* fix up BUILD file to pass the flag