Resolve "CI: test_mg_restart_with_lima_disabled_counters fails"

Closes #3878 (closed)

test_mg_restart_with_lima_disabled_counters tests whether disabled MG counters are still disabled after restarting the session.

To ensure we test a fresh restart, the test first checks that the available counters in the measurement group are empty.

The measurement group gets available counters from global state.

In the original test we relied on garbage collection to clear the global state. In this MR we clear the global state explicitly.