tests: dpkt-gated tests have never run on any CI path (28 methods, 6 files) #729

Describe the bug

dpkt is declared only in the DPKT extra (pyproject.toml:135) and in all (:217). No CI job installs either — the three install lines in .github/workflows/unit-tests.yml are .[test] (:82) and .[test,Scapy] (:128, :247). So every @unittest.skipUnless(HAS_RUNTIME and HAS_DPKT, ...) test has never executed on any CI path, in any job, on any Python version. They report as skips, which is indistinguishable from a pass in the summary line.

Measured by AST — 28 test methods across 6 files:

The last two are the sharp end: they live in tests/integration/, which is the job whose entire purpose is exercising the runtime engines.

Reproduction

$ grep -n 'pip install -e' .github/workflows/unit-tests.yml
82:          python -m pip install -e '.[test]'
128:          python -m pip install -e '.[test,Scapy]'
247:          python -m pip install -e '.[test,Scapy]'
$ python -c "import re;t=open('pyproject.toml').read();print([n for n,b in re.findall(r'^(\w+)\s*=\s*\[(.*?)\]',t,re.S|re.M) if 'dpkt' in b])"
['DPKT', 'all']

Expected behavior

Either the DPKT extra joins the integration/gate install line so those 28 tests actually run, or the skip is made deliberate and visible so nobody reads it as coverage.

Additional context

Pre-existing and unrelated to #715 — found incidentally while measuring #727's skip-set. Scapy is already installed this way (:128), so the precedent for adding an engine extra to that line exists. Note HAS_RUNTIME gates the same tests, so adding dpkt alone may not be sufficient; check what HAS_RUNTIME requires before assuming a one-word fix.