Tests: amrex.the_arena_init_size=0

`amrex.the_arena_init_size=0` is default on CPU but not GPU.
In order to run tests in parallel on the same GPU, we need
to change the default so the first test does not allocate
3/4th of the memory of the GPU (to speed up later allocations
in a pre-allocated heap).

Author: ax3l

tests/conftest.py

@@ -43,6 +43,10 @@ def amrex_init(tmpdir):
                 # "abort_on_out_of_gpu_memory=1",
                 # avoid managed memory unless explicitly used
                 "amrex.the_arena_is_managed=0",
+                # allocate GPU memory on-demand instead of pre-allocating 3/4th
+                # to enable parallel test runs on the same GPU
+                # https://amrex-codes.github.io/amrex/docs_html/RuntimeParameters.html?highlight=arena#memory
+                "amrex.the_arena_init_size=0",
             ]
         )
         yield