Follow Damian's advice to use list(document) to get Stem `RelayDescriptor`s.

See https://bugs.torproject.org/9380#comment:16.
Fix parseNetworkStatusFile() to actually return a dict.

Previously, it was returning a BridgeNetworkStatusDocument class.
Add utility function to test_parse_descriptors for writing test descriptor files.

 * ADDS new method:
   test_parse_descriptors.ParseDescriptorsTests.writeTestDescriptorsToFile().
Add test method to encsure that parseNetworkStatusFile() returns a dict.
Change unittest; parseNetworkStatusFile() should return `RouterStatusEntryV2`s.
Add unittest for parseNetworkStatusFile() with two descriptors.
Add unittest explicitly for return type of parseBridgeExtraInfoFiles().
Add test to check return of parseNetworkStatusFile() has `RouterStatusEntryV2`s.
Fix fingerprints in two tests for parseNetworkStatusFile().
Add another unittest for parseBridgeExtraInfoFiles() return type.
Fix some docstring stylings in test_parse_descriptors.
Change unittest to demand {(fpr, router),...} is returned after deduplicate().
Fix faulty unittests which expected parsed extrainfos to have an `address`.
This test was supposed to test deduplication. Now it does.
Add docstring to `bridgedb.parse.descriptors.deduplicate()`.
Add more info to docstring for `b.p.descriptors.parseBridgeExtraInfoFiles()`.
Rewrite `b.p.descriptors.deduplicate()` to not modify while indexing.