* Initial libxml2 tests (using executables)

* Expanded libxml2 tests using installed bins

* Refactored/generalized _run_tests

Existing test methods run via callbacks at install time when run with `spack install --run-tests`
These methods are tied into the package build system, and cannot be run arbitrarily
New naming scheme for these tests based on the build system phase after which they should be run
The method name `test` is now reserved for methods run via the `spack test` command

* Added install test for perl, including use statements

* Preliminary m4 smoke tests

* Initial patchelf smoke tests

Restores the fetching progress bar sans failure outputs; restores non-debug reporting of using fetch cache for installed packages; and adds a unit test.

* Add status bar check to test and fetch output when already installed

* Adding v6.6 (latest stable)

* There is no '-' in Quantum ESPRESSO

Co-authored-by: Filippo Spiga <fspiga@nvidia.com>

Some of the feature flags are named differently and clwb is missing on
my i7-1065G7. cascadelake and cannonlake might have similar problems but
I do not have access to those architectures to test.

- add cuda variant, enabled by default, but conflicting with
  strumpack@:3.9.999
- add zfp variant, enabled by default, but conflicting with
  strumpack@:3.9.999
- update minimum CMake version to 3.11
- for version 4.0.0:, do not use mpi wrappers. v4.0.0 uses CMake
  MPI targets
- for version 4.0.0, add dependency on butterflypack@1.2.0:
- remove versions 3.1.0 and older
- make parmetis variant True by default
- add TODO for slate variant (spack package not ready yet)