bootc: Move install tests into a dedicated install test suite, dedup install-in-test code #249

From cgwalters' review of bootc-dev/bootc#2500 (tmt/tests/booted/test-52-install-composefs-native.nu):

• Tests that build an image and run bootc install against a loopback disk (plan-52-install-composefs-native, and install-outside-container, install-to-filesystem-var-mount and similar) don't depend on the booted host environment. They belong in an explicit "install tests" suite rather than the booted tmt suite.
• The install-in-test code (build a derived image, podman run --privileged ... bootc install to-disk --via-loopback, mount the disk and inspect it) is duplicated across these tests and should be shared.

Generated-by: https://github.com/cgwalters/#llms