Allow example tests to be run on subprocesses

[leofang]

Right now we just exec all tests in the main test process

cuda-python/cuda_core/tests/example_tests/utils.py

Line 35 in 213baf4

exec(script, env if env else {})

which is fine if there is no subtle bugs in the examples. However, it is not the end user experience (ex: from user perspective CUDA contexts are implicitly created/tear down between each sample runs). We should allow configuring it in the future.