vlog: Add TEST_LOG env var with RUST_LOG-style syntax
Update Vlog.setup_test to:
- Default to Debug level (Alcotest captures output anyway)
- Support RUST_LOG-style syntax: TEST_LOG=level,src:level,...
Examples: TEST_LOG=warning, TEST_LOG=warning,conpool:debug
Update all test files to use Vlog.setup_test:
- ocaml-requests: all 17 test files
- ocaml-claudeio: test_claude.ml
- ca-certs: tests.ml
Document TEST_LOG in ocaml-testing skill with comparison to
other ecosystems (RUST_LOG, GOLOG_LOG_LEVEL, pytest, DEBUG).