[VC-35738] Move the agent --one-shot test to the cmd package

[wallrj]

Instead of duplicating the log initailization and exit behaviour of the Execute function,
I moved this test to the cmd package where I can simply run the Execute function directly.

This simplifies the test and avoids the test and implementation getting out of sync.

$ go test ./cmd/... -v
=== RUN   TestAgentRunOneShot
    agent_test.go:41: Running child process
    agent_test.go:59: STDOUT
        I1107 13:27:46.472452  276234 logs.go:153] "Preflight agent version: development ()" source="agent"
        I1107 13:27:46.472578  276234 logs.go:153] "Using the Jetstack Secure API Token auth mode since --api-token was specified." source="agent"
        I1107 13:27:46.472594  276234 run.go:119] "Healthz endpoints enabled" logger="APIServer" addr=":8081" path="/healthz"
        I1107 13:27:46.472605  276234 run.go:123] "Readyz endpoints enabled" logger="APIServer" addr=":8081" path="/readyz"
        I1107 13:27:46.472668  276234 run.go:460] "Starting" logger="APIServer.ListenAndServe" addr=":8081"
        I1107 13:27:46.473314  276234 logs.go:153] "Reading data from local file: testdata/agent/one-shot/success/input.json" source="agent"
        I1107 13:27:46.473378  276234 logs.go:153] "Data saved to local file: /dev/null" source="agent"
        I1107 13:27:46.473393  276234 run.go:474] "Shutting down" logger="APIServer.ListenAndServe" addr=":8081"
        I1107 13:27:46.473439  276234 run.go:489] "Shutdown complete" logger="APIServer.ListenAndServe" addr=":8081"

    agent_test.go:60: STDERR

--- PASS: TestAgentRunOneShot (0.02s)
PASS
ok      github.com/jetstack/preflight/cmd       0.045s