Testing: fix unintended interactions between tests (#16003)

* For tests that use the real Spack package repository, the config
  needs to avoid using MPI providers that are not intended to be
  installed by Spack. Without this, it is possible that Spack tests
  which concretize the MPI virtual will end up trying to use an
  implementation that it shouldn't (e.g. one that is always
  provided externally). See #15666 for an example.
* The mutable_config test fixture was not initializing the scope
  roots to the right directories (so the resulting config was empty).
* The current_host fixture in the concretize.py tests was using the
  config fixture rather than mutable_config, and was polluting the
  config cache for other tests.
* One test in concretize.py was clearing a nonexistent cache
  (PackagePrefs._packages_config_cache). This reference has been
  removed.
* The test 'test_preferred_compilers' was was depending on cross
  test config pollution to succeed. The initial spec before
  concretization has been updated to updated to be explicit about
  the desired result.

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