Add comprehensive testing infrastructure

- Testing strategy document
- Unit tests for GPS validator
- Integration test conftest with fixtures
- Sample .sub files and manifests
- Comprehensive Termux testing guide
- Test directory structure

tests/fixtures/manifests/valid_single.json

@@ -0,0 +1,13 @@
+{
+    "session_uuid": "test-session-001",
+    "captures": [
+        {
+            "filename": "valid_key_433mhz.sub",
+            "latitude": 40.7128,
+            "longitude": -74.0060,
+            "accuracy": 5.0,
+            "altitude": 10.5,
+            "timestamp": "2026-01-12T10:00:00Z"
+        }
+    ]
+}