CMake: Replace `AMReX_TEST_TYPE` with Labels

[ax3l]

Instead of a configure-time option, we can at the runtime of CTest select tests by adding labels to them.

Reference:

• https://cmake.org/cmake/help/latest/prop_test/LABELS.html
• https://cmake.org/cmake/help/latest/manual/ctest.1.html#label-matching

Example that added a label to slow tests in ImpactX:

• https://github.com/ECP-WarpX/impactx/pull/520