Fix RUBY-2287 Max staleness spec tests do not correctly assert all requirements

[neilshweky]

All I did was add a check that suitable servers was empty if available servers was empty. I checked that if there was an available Primary or Secondary server and the suitable servers was empty that it would fail.

[egiurleo]

spec is actually an object representing the yaml file that the test is based on, so I think you'll want to use this as the expected value in your tests.

Take a look at line 128 for an example of something similar -- in_latency_window is derived from spec.in_latency_window. We take the results of server selection and make sure that they match the value specified by the yml file.

[p-mongo]

I ended up moving a lot of code around to make the tests test the driver instead of testing themselves.

Behavior changes:

• I believe the driver did not validate deployment's staleness support in some cases. Now the validation happens in the same place for all topology types.
• select in server selector classes was renamed to select_in_replica_set which makes more sense to me based on what it does. It so happens that select method is still there which is the Kernel.select (the file descriptor/socket selection one). This annoyingly causes code that uses the old method to hang when select is now invoked. I believe this change is in private API although the methods weren't marked as such (they are now).
• Selectable module is now gone, its functionality was moved to base server selector. This shouldn't affect the public API.

Test changes:

• Max staleness spec tests were redone to no longer hack their way into greenness. Some tweaks to driver code were needed to provide a list of suitable servers to the tests.
• Spec test runner reorganized to be more linear when defining various cases, to guard against case lossage.
• Suitable servers assertions demanded by the spec are now implemented.
• I've done some spot checks and spec tests now fail when yaml files are edited to be bogus.
• Server selector and max staleness use the same exact runner instead of having close-but-not-quite identical copy-pastes of it.
• Spec test definition moved to the runner since it's now shared between two tests.

[p-mongo]

https://jira.mongodb.org/browse/RUBY-2287