Modifications:
* Fix a severe bug where a spliced spec has the same hash as the original build spec
* Removed CacheManager in favor of install_mockery. The latter sets up a temporary store, and removes it at the end
of the test.
* Deleted a couple of helper functions e.g. _has_dependencies
* Checked that SolverException is raised, rather than Exception (more specific)
* Extended, and renamed the splicing_setup fixture (now called install_specs)
* Added more specific assertions in each test
One test is currently flaky, due to some instability when sorting multiple specs. It's currently marked xfail
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