test: use EphemeralTestnet singleton in nexus-watcher tests

[ok300]

This PR simplifies test setup in the nexus-watcher suite by only initializing the test stack and the EphemeralTestnet once, then re-using them across the nexus-watcher tests.

[SHAcollision]

IMO good to merge. But i thought it might be relevant to raise for you this Codex comment:

3. create_external_test_homeserver() depends on global initialization order
   • The helper now errors if SHARED_TESTNET is not initialized, which is correct but fragile. If any future test calls this helper without first going through WatcherTest::setup(), it will return an error (SHARED_TESTNET not initialized) and fail unexpectedly.
   • Recommendation: either make create_external_test_homeserver() lazily initialize the testnet, or document the required initialization order clearly.