Tests: Add tests/README.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/README.md

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+# PythonBPF Test Suite
+
+## Quick start
+
+```bash
+# Activate the venv and install test deps (once)
+source .venv/bin/activate
+uv pip install -e ".[test]"
+
+# Run the full suite (IR + LLC levels, no sudo required)
+make test
+
+# Run with coverage report
+make test-cov
+```
+
+## Test levels
+
+Tests are split into three levels, each in a separate file:
+
+| Level | File | What it checks | Needs sudo? |
+|---|---|---|---|
+| 1 — IR generation | `test_ir_generation.py` | `compile_to_ir()` completes without exception or `logging.ERROR` | No |
+| 2 — LLC compilation | `test_llc_compilation.py` | Level 1 + `llc` produces a non-empty `.o` file | No |
+| 3 — Kernel verifier | `test_verifier.py` | `bpftool prog load -d` exits 0 | Yes |
+
+Levels 1 and 2 run together with `make test`. Level 3 is opt-in:
+
+```bash
+make test-verifier   # requires bpftool and sudo
+```
+
+## Running a single test
+
+Tests are parametrized by file path. Use `-k` to filter:
+
+```bash
+# By file name
+pytest tests/ -v -k "and.py" -m "not verifier"
+
+# By category
+pytest tests/ -v -k "conditionals" -m "not verifier"
+
+# One specific level only
+pytest tests/test_ir_generation.py -v -k "hash_map.py"
+```
+
+## Coverage report
+
+```bash
+make test-cov
+```
+
+- **Terminal**: shows per-file coverage with missing lines after the test run.
+- **HTML**: written to `htmlcov/index.html` — open in a browser for line-by-line detail.
+
+```bash
+xdg-open htmlcov/index.html
+```
+
+`htmlcov/` and `.coverage` are excluded from git (listed in `.gitignore` if not already).
+
+## Expected failures (`test_config.toml`)
+
+Known-broken tests are declared in `tests/test_config.toml`:
+
+```toml
+[xfail]
+"failing_tests/my_test.py" = {reason = "...", level = "ir"}
+```
+
+- `level = "ir"` — fails during IR generation; both IR and LLC tests are marked xfail.
+- `level = "llc"` — IR generates fine but `llc` rejects it; only the LLC test is marked xfail.
+
+All xfails use `strict = True`: if a test starts **passing** it shows up as **XPASS** and is treated as a test failure. This is intentional — it means the bug was fixed and the test should be promoted to `passing_tests/`.
+
+## Adding a new test
+
+1. Create a `.py` file in `tests/passing_tests/<category>/` with the usual `@bpf` decorators and a `compile()` call at the bottom.
+2. Run `make test` — the file is discovered and tested automatically at all levels.
+3. If the test is expected to fail, add it to `tests/test_config.toml` instead of `passing_tests/`.
+
+## Directory structure
+
+```
+tests/
+├── README.md                  ← you are here
+├── conftest.py                ← pytest config: discovery, xfail/skip injection, fixtures
+├── test_config.toml           ← expected-failure list
+├── test_ir_generation.py      ← Level 1
+├── test_llc_compilation.py    ← Level 2
+├── test_verifier.py           ← Level 3 (opt-in, sudo)
+├── framework/
+│   ├── bpf_test_case.py       ← BpfTestCase dataclass
+│   ├── collector.py           ← discovers test files, reads test_config.toml
+│   ├── compiler.py            ← wrappers around compile_to_ir() + _run_llc()
+│   └── verifier.py            ← bpftool subprocess wrapper
+├── passing_tests/             ← programs that should compile and verify cleanly
+└── failing_tests/             ← programs with known issues (declared in test_config.toml)
+```
+
+## Known regressions (as of compilation-context PR merge)
+
+Three tests in `passing_tests/` currently fail — these are bugs to fix, not xfails:
+
+| Test | Error |
+|---|---|
+| `passing_tests/assign/comprehensive.py` | `TypeError: cannot store i64* to i64*` |
+| `passing_tests/helpers/bpf_probe_read.py` | `ValueError: 'ctx' not in local symbol table` |
+| `passing_tests/vmlinux/register_state_dump.py` | `KeyError: 'cs'` |
+
+Nine tests in `failing_tests/` were fixed by the compilation-context PR (they show as XPASS). They can be moved to `passing_tests/` when convenient:
+
+`assign/retype.py`, `conditionals/helper_cond.py`, `conditionals/oneline.py`,
+`direct_assign.py`, `globals.py`, `if.py`, `license.py` (IR only), `named_arg.py`,
+`xdp/xdp_test_1.py`