Add integration tests for authkey deployment via system policies

In https://github.com/tailscale/tailscale/pull/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