Add more tests.

Tests functionality related to discussions in #38.

There are now tests for ~3 different ways to interface with TLDExtract:
1. 'normal' invocation (nearly equivalent to using `tldextract.extract`,
however wanted to specify `cache_enabled=False` in order to silo the tests)
2. live http connection against PSL (`cache_enabled=False` and `fetch=True`)
3. using custom nonsense suffix list containing suffixes `foo`, `bar`, and
`baz`. this is to test the `suffix_list_url` argument being aimed at a local
file.