test(e2e): consolidate selector tests, add round-robin/fifo/backup failover

Merge parallel_selector_test.go into selector_test.go and add e2e coverage
for the round-robin + fail filter, fifo sticky fallback, and backup filter
strategies. Each test drives requests through a hop with one dead node and
asserts the selector marks and skips it so traffic converges on the live node.

Co-Authored-By: Claude <noreply@anthropic.com>

tests/e2e/README.md

@@ -35,7 +35,7 @@ tests/e2e/
 │   └── udp_echo.py             # UDP echo server (reflects payloads)
 ├── testdata/                   # config files or data files for running cases
 ├── shadowsocks_test.go         # Shadowsocks protocol tests
-└── parallel_selector_test.go   # Parallel node selector tests
+└── selector_test.go            # Node selector tests (round-robin, fifo, backup, parallel)
 ```
 
 ### How it works