Set an environment variable with current test

[nicoddemus]

When executing tests in CI, sometimes a test may hang indefinitely. One can look at the output using -v but that might not always be reliable.

If pytest would set a specific environment variable with the test it is executing, one can use a process monitoring library like psutil to figure out which test is executing. This could be a variable named something like PYTEST_CURRENT_TEST which is set to the test node id. Alternatively, we could also add if the test is in the setup, call or teardown phase.

The implementation should be trivial and can be really helpful when diagnosing CI problems.

This was originally reported in pytest-xdist (pytest-dev/pytest-xdist#189), but @RonnyPfannschmidt correctly identified this should go into core pytest instead.