Convert test_bootstrap.py to unittest so discover collects it

[ptr727]

scripts/tests/test_bootstrap.py defined its ten checks as module-level test_* functions with its own check()/failures collector and main() runner, no unittest.TestCase. CI's only invocation, validate-task.yml's unittest discover -s scripts/tests, loads TestCase subclasses from each module it imports; a bare test_* function is not one, so none of the loader-invariant or spec-to-installer coverage checks ran.

What changed

Converted the file to unittest.TestCase, matching the other six files under scripts/tests/:

• TestLoaderInvariant — the two loaders each read exactly one path into the fetched tree, and need no Python.
• TestSpecCoverage — every tool spec/host-tools.json requires is one the platform installers can provide, and every floored tool carries a total remedy mapping.
• TestScriptPresence — every script a loader hands control to is present and, on Linux, tracked executable; the Windows scripts and bootstrap.ps1 carry no shebang.
• TestHarness — floors the collected test count, matching the sibling files' pattern.

Dropped the check()/failures global collector for self.assert* and self.subTest, so a loop over multiple items (tools, platforms, forbidden paths) reports each failing item as its own subtest instead of one lumped list. Added the shebang and executable bit the other six sibling files carry.

Verification

• python3 scripts/tests/test_bootstrap.py — 11 tests, OK.
• python3 -m unittest discover -s scripts/tests — 686 tests, OK (previously ran none of this file's checks; confirmed by grepping the -v output for bootstrap before this change).
• uvx coverage@latest run --source=scripts,spec,host-setup -m unittest discover -s scripts/tests — the exact CI invocation — 686 tests, OK.
• uvx ruff@latest check ., uvx ruff@latest format --check ., uvx mypy@latest — clean.
• python3 scripts/prose_lint.py, python3 scripts/repo_gate.py — clean.
• docker run ... mstruebing/editorconfig-checker:latest — clean on the changed file (only gitignored local tool caches flagged, not part of the change).
• Fault injection: appended a forbidden $TREE/spec/... read to a scratch copy of host-setup/bootstrap.sh and reran — TestLoaderInvariant failed with the expected message, confirming the converted checks actually catch a violation rather than passing vacuously. Reverted before committing.

Fixes #754

[Copilot]

Pull request overview

This pull request fixes a CI test-discovery gap by converting scripts/tests/test_bootstrap.py from a custom module-level test_* + manual runner pattern into unittest.TestCase classes so python -m unittest discover -s scripts/tests actually collects and runs the checks (loader invariant + spec-to-installer coverage).

Changes:

• Refactors the bootstrap checks into unittest.TestCase suites using assert* + subTest, replacing the prior global failures collector and main() runner.
• Adds a small “harness” test to assert the module collects a minimum number of test cases (guarding against silent collection failures).
• Adds a shebang and enables direct execution via unittest.main(verbosity=2).

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