Fix error when using simulator test with pre-defined environment variable

Fix error when using simulator test with pre-defined environment variable

The simulator test compares the results of functionality implemented in
both the legacy and the modular simulator by running two simulations using
the environment variables GMX_DISABLE_MODULAR_SIMULATOR and
GMX_USE_MODULAR_SIMULATOR to cover both code paths. If one of these
variables was already present in the test sytem, the tests would fail with
an error.

This change backs up the current state of both variables, disables them,
runs the tests, and resets them to their previous state (if any).