Add tests for installing after init_osname was already called (i.e. default sysroot dir exists)

[ckyrouac]

Spawned from https://github.com/bootc-dev/bootc/pull/1264#issuecomment-2796925908

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Add support for something like fail points Add a failpoint around this case of post-stateroot init, pre-pull Add a test case into tests-integration/src/install.rs

https://github.com/coreos/rpm-ostree/pull/4259