Improve test suite

Improves the existing test suite by splitting the single test file into multiple files.

Also adds the ability for other people to use the tests with some example data.

tests/fixtures/README.md

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+# Synthetic network fixture
+
+`synthetic_history_network.json` is the committed end-to-end packet fixture for
+the exporter test suite. It contains UDP payloads for Permanent Board and Arc
+history requests/responses, plus isolated protocol edge cases.
+
+The fixture is safe to publish:
+
+- endpoints use the RFC 5737 documentation ranges `192.0.2.0/24` and
+  `198.51.100.0/24`;
+- packet and history timestamps are deterministic synthetic values;
+- it contains no game user UID, account/session data, cookies, tokens, or raw
+  capture metadata;
+- the test suite checks every payload with the UID extractor before accepting
+  it.
+
+Tests load the fixture through helpers in `tests/support.py`. The `replay`
+scenario is fed through `LiveHistorySession`; `protocol_sample` packets cover
+points gifts, chase rewards, batched pages, and bit-packed responses.
+
+Do not replace this file with a real `.pcap`, `.flows`, or exported account
+history. Add new cases by constructing the smallest relevant payload, replacing
+all timestamps and endpoints, and extending the privacy assertions.