cxx: new test API (#45462)

* cxx: new test API
* gcc: provide cxx
* default providers:  cxx provided by gcc
* cxx: cleanup stand-alone test
  - test_c -> test_cxx
  - simplify compilation and execution
  - corrected output checks

---------

Co-authored-by: Tamara Dahlgren <dahlgren1@llnl.gov>

etc/spack/defaults/packages.yaml

@@ -20,6 +20,7 @@ packages:
       awk: [gawk]
       armci: [armcimpi]
       blas: [openblas, amdblis]
+      cxx: [gcc]
       D: [ldc]
       daal: [intel-oneapi-daal]
       elf: [elfutils]