Add some test helpers for common patterns

Updates the tests to:

1. Always require the `test_helper` file
2. Add `assert_valid` and `refute_valid` to replace the common `assert(Validator.validate(...))` pattern
3. Add `schema_fixture_path` and `data_fixture_path` to replace the common `File.join(....)` pattern
4. Use those helpers via mechanical search & replace
5. A few manual tweaks for clarity (test_extends_and_additionalProperties.rb was a little more complicated than it needed to be, extract methods if the same fixtures were used for an entire file, etc)

I thought I should get some feedback before I sink any real time into this.
Should this just require lib/json_schema.rb? I'm not 100% sure here whether lib/json_schema.rb or the json_schema gem comes first in the load path.