Run unittest discovery in the coverage target

[x0Lazarus]

Follow up on #54's move to unittest discovery: make coverage still executes tests.py directly, which now defines the tests without running them. This lets the CI test command succeed without exercising the suite.

Invoke unittest through coverage so the target runs both unit tests and the load_tests doctests, and returns a failure when a test fails.

Validated on Windows / Python 3.12 by executing the recipe commands directly; GNU make was unavailable:

• The unchanged suite runs 28 passing tests, including doctests, with 100% statement/branch coverage.
• A deliberately failing test in a temporary copy was ignored by the old command (exit 0) and detected by the corrected command (exit 1).
• flake8 passes.

The hosted Ubuntu / Python 3.10-3.14 matrix remains unverified locally.

[Copilot]

Copilot review overview

🟢 Approval recommended

The coverage target now correctly discovers tests and propagates failures.

Review effort: Balanced
Findings: None

What changed in this PR

Updates the coverage target to run the discovered test suite and doctests.

Changes:

• Runs unittest through coverage instead of executing tests.py directly.

File	Description
makefile	Corrects the coverage test invocation.

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[stefankoegl]

Thanks!