Adds kill_on_exit to _AgentRunner in testing

[jlashner]

Description

Adds the option to kill agents in tests using SIGKILL instead of SIGINT.

Motivation and Context

In my development of of the HWP emulator-based integration tests (simonsobs/socs#802) I'm finding that, though the test are working, shutting agents down with a single interrupt command sometimes does not work, and is fairly slow (~10 sec per agent for the agents that do work). I believe this could be something related how and when socket connections are closed by the emulator but I'm not entirely sure.

I have found that using a SIGKILL instead of SIGINT to shutdown agents is much faster and more reliable, and I don't really think we care about clean shutdowns at the end of states, so I think this is a good option for some tests, such as the HWP integration tests which use many different agents.

How Has This Been Tested?

This has been tested by running HWP integration tests from the PR above locally.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[BrianJKoopman]

Looks good to me!