Add comprehensive tests for machine/process models, orchestrator, and search backends

This adds new test coverage for previously untested areas:

Machine module (archivebox/machine/tests/):
- Machine, NetworkInterface, Binary, Process model tests
- BinaryMachine and ProcessMachine state machine tests
- JSONL serialization/deserialization tests
- Manager method tests

Workers module (archivebox/workers/tests/):
- PID file utility tests (write, read, cleanup)
- Orchestrator lifecycle and queue management tests
- Worker spawning logic tests
- Idle detection and exit condition tests

Search backends:
- SQLite FTS5 search tests with real indexed content
- Phrase search, stemming, and unicode support
- Ripgrep search tests with archive directory structure
- Environment variable configuration tests

Binary provider plugins:
- pip provider hook tests
- npm provider hook tests with PATH updates
- apt provider hook tests

archivebox/plugins/npm/tests/__init__.py

@@ -0,0 +1 @@
+"""Tests for the npm binary provider plugin."""