fix[HostTargetSessionObserverTest.cpp]: call unsubscribe to cleanup resource (#45701)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/45701

# Changelog: [Internal]

One weird thing is that this test actually never runs for Android, so its only compiled. I am not sure why exactly this one produces an error during execution, given that the next test is almost identical.

Nothing valuable from logcat, just a `SIGSEGV`. Since its something with memory, I've tried calling `unsubscribe`, same as in the next test to free memory before the test tear down (which should not run, because this test doesn't run).

Reviewed By: dmytrorykun

Differential Revision: D60282464

fbshipit-source-id: 2c7760f02c1128083f28651fe8ecd0f3cee27715