Add integration tests for authkey deployment via system policies

[agottardo]

In #13061, we added the ability to use system policies (MDM) to deploy an authkey.

tailscale/tstest/integration/integration_test.go already contains a test (TestConfigFileAuthKey) for similar logic (deploying an authkey using a text file), but it is currently impossible to write an equivalent test because there is no way to mock values in the syspolicy package across the process boundary.

We need the ability to define mock syspolicies in integration_test.go to fully test this feature (and other policies moving forward).

cc @nickkhyl who is working on related things

[nickkhyl]

@agottardo: I think we can add an envvar-based policy source that would have lower precedence than platform-specific policy sources (aka GP/MDM). This could be useful for both testing and certain production scenarios.

#13113 is currently under review. Hopefully, I'll be able to address the feedback, get it merged, and submit the next PR for review sometime this week. It won't be the last in the series, but should enable us to create and utilize additional policy sources.