Clean ups in --test-mode code

[LalatenduMohanty]

There were some followup clean ups suggested by @dhellmann during review of #865. This issue tracks these.

1. Use existing tmp_context fixture

• File: tests/test_bootstrap_test_mode.py
• Comment: There's already a fixture for making a temporary context. If you name an argument to a test method tmp_context you get a context in a temporary directory that is also managed by pytest.
• Reference: https://github.com/python-wheel-build/fromager/blob/main/tests/conftest.py#L24

2. Move resolution step from _bootstrap_impl to bootstrap

• File: src/fromager/bootstrapper.py (line ~232)
• Comment: "We wouldn't need this logic [getting version from cache] if we move the resolution step out of _bootstrap_impl into this function where we can see whether it resolved in the first place."

3. Move _track_why context manager to bootstrap

• File: src/fromager/bootstrapper.py (line ~295)
• Comment: "If we move the resolution logic up into bootstrap then this context manager can also move there because we will have the resolved version."

4. Consider force_prebuilt flag to eliminate _handle_test_mode_failure

• File: src/fromager/bootstrapper.py (line ~848)
• Comment: "If we add a force_prebuilt flag to _bootstrap_impl then we wouldn't need this function and we could avoid some duplication of the logic for handling pre-built wheels."