Add .exit command to input in TestProc to ensure that process will shut down

[RebeccaMahany]

New theory about test flakiness:

• loadExtensions is actually a blocking operation, since it calls extensionManagerServer.Start()
• This means that StartProcess doesn't actually return until the process has shut down
• My theory is that on Ubuntu, the process is actually running happily. This is supported by logs in recent failures (https://github.com/kolide/launcher/actions/runs/15878752198/job/44773402854?pr=2334), showing that the osquery process is responding to queries with results.

I added some documentation to interactive.go to make it clearer that loadExtensions isn't just loading extensions but also running the server until it is shut down.

To attempt to address the test flakiness, I have added a .exit command to our input, to ensure that the process will shut down so the test can proceed.

[zackattack01]

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