Tests fail on Windows: POSIX inline env-var syntax (PYTHONPATH=...) not portable #31

Problem

Three tests pass the environment to a subprocess using POSIX inline syntax (PYTHONPATH=... <command>). PowerShell and cmd do not understand this form, so the subprocess runs without PYTHONPATH set and the tests fail.

Running the full suite on Windows yields:

116 passed, 1 skipped, 3 failed

On POSIX (Linux/macOS) the suite is fully green (124 passed), so this is a test-portability issue, not a defect in becwright itself.

Where it was observed

Surfaced while reviewing PR #30. The PR's own changes are unrelated to the failures; they are pre-existing.

Suggested fix

Stop injecting the variable on the command line. Instead pass the environment explicitly to the subprocess, e.g.:

• build an env dict and pass it via subprocess.run(..., env=env), or
• use monkeypatch.setenv("PYTHONPATH", ...) before spawning,

so the tests run identically on POSIX and Windows shells.

Scope

Test-only change. No change to the engine, rules format, or check logic.