Add Mix alias to test on all protocols

[whatyouhide]

Right now it's pretty clunky to run tests on different native protocols. You have to do something like this:

CASSANDRA_NATIVE_PROTOCOL=v3 mix test test/my_test.exs
CASSANDRA_NATIVE_PROTOCOL=v4 mix test test/my_test.exs
CASSANDRA_NATIVE_PROTOCOL=v5 mix test test/my_test.exs

Our CI matrix is also pretty big, and part of that is to test on multiple protocols:

However, every C* connection can specify its own native protocol, which means that for any given C* version we can run all protocol tests that that C* version supports.

We can add a new Mix alias for this:

mix test.native_protocols

which is essentially gonna loop through the supported protocols.

[whatyouhide]

I did this in f774ba5.