Improve the skip condition for cuFile binding tests

[leofang]

During the bring-up #684 we noticed that some cuFile bindings require the internal thread pool to be activated, but on some systems they are not activated by default (for reasons I don't fully understand yet). We had to provide a custom json file pointed to by an env var CUFILE_ENV_PATH_JSON to enforce the activation and make the tests pass on this local system.

We should consider either or both of the following approaches:

1. Encode the activation criteria as a pytest skip decorator, so that the tests that need the threadpool to pass would be skipped
2. Add a cufile.json file to the test directory (either statically or dynamically) and set up CUFILE_ENV_PATH_JSON to force activating the thread pool

[leofang]

cc @sourabgupta3 for vis