tests: refactor tests to avoid persistent global state

Previously, fixtures like `config`, `database`, and `store` were
module-scoped, but frequently used as test function arguments.  These
fixtures swap out global on setup and restore them on teardown.  As
function arguments, they would do the right set-up, but they'd leave the
global changes in place for the whole module the function lived in.  This
meant that if you use `config` once, other functions in the same module
would inadvertently inherit the mock Spack configuration, as it would
only be torn down once all tests in the module were complete.

In general, we should module- or session-scope the *STATE* required for
these global objects (as it's expensive to create0, but we shouldn't
module-or session scope the activation/use of them, or things can get
really confusing.

- [x] Make generic context managers for global-modifying fixtures.

- [x] Make session- and module-scoped fixtures that ONLY build filesystem
  state and create objects, but do not swap out any variables.

- [x] Make seeparate function-scoped fixtures that *use* the session
  scoped fixtures and actually swap out (and back in) the global
  variables like `config`, `database`, and `store`.

These changes make it so that global changes are *only* ever alive for a
singlee test function, and we don't get weird dependencies because a
global fixture hasn't been destroyed.

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