Run bootstrap.ps1's Main Flow End to End Against a Stub Download and Stub Tools #2587

Follow-up to #1767, the PowerShell half of its option 1.

The bash loader's whole main flow is now run end to end in tests/test_bootstrap.py (TestKeptTreeEndToEnd): bootstrap.sh --host against a scratch directory, a stub curl serving a local tarball, and stub tools failing at a chosen stand-up step, asserting the kept tree left on disk.

Still owed, in the windows lane: the same for host-setup/bootstrap.ps1. Run its main flow with a stubbed download and stub tools in the fetched tree failing at each stand-up step, and assert the properties the bash class asserts:

• a stand-up failing before the skills step leaves the previous kept tree loading, with no .new or .old beside it;
• a successful run leaves only the new tree;
• the previous tree is untouched until the skills step, and replaced before the installer runs.

TestPowerShellKeptTreeHandling already covers the loader's functions in isolation, so this is the whole-flow counterpart only.