* Fix issue with HDF5_VOL_ALLOW_EXTERNAL CMake variable

* Add initial API test workflow

* Initialize parallel testing with MPI_THREAD_MULTIPLE when testing API

* Add CMake variable to allow specifying a VOL connector's package name

* Remove call to MPI_Init in serial API tests

While previously necessary, it now interferes with VOL connectors that
may need to be initialized with MPI_THREAD_MULTIPLE