Modify tests to support multiple graph implementations

[drone29a]

I am planning to update the tests so that graph creation functions may be provided to create graphs which adhere to Loom graph protocols. This facilitates testing third-party (not included in Loom) graph implementations.

The core.matrix library does something similar, but also includes a mechanism to specify a global implementation. I do not believe Loom benefits from this global setting, but tests should still be organized to provide easy coverage for all Loom graph implementations.

I will do a quick review of how other projects have tackled this problem and then either use test functions which take extra parameters (the graph creation functions) or dynamic binding in order to expose graph creation fns to tests.

If someone is opposed or has additional recommendations, please comment!

[drone29a]

I'm thinking something like this, which passes in an instance for testing.

https://github.com/mikera/core.matrix.testing

We could keep it as part of the main loom project for now, but packaging it separately (like core.matrix) seems reasonable too. Any pros/cons?