[VC-43403] Fix cmd tests

[wallrj-cyberark]

The test in cmd/ was not being run in CI so we didn't notice that it was failing.
It has been failing since we merged #670, because there we added the clusteruid detection to the agent startup code, and it unconditionally attempts to connect to a Kubernetes cluster, even when the agent is being run with --input-file and --output-file.
The --input-file option is used for testing the pushing of data to TLSPK or Jetstack Secure, in isolation.
The --output-file option is used for testing the collection of data, in isolation.
And theoretically, a user might choose to collect data from "air-gapped" (non-Internet-connected) clusters, using --output-file,
then use another instance of the agent to push the data to TLSPK from an Internet connected server.

I intend to move the clusteruid detection to be part of the data gathering process, in another PR: #687.

Here's the test failing, after I added the test to make test-unit:

• https://github.com/jetstack/jetstack-secure/actions/runs/17103417847/job/48505887344

=== FAIL: cmd TestAgentRunOneShot (3.00s)
    agent_test.go:41: Running child process
    agent_test.go:59: STDOUT
        I0820 15:46:20.507633    2589 run.go:59] "Starting" logger="Run" version="development" commit=""
        I0820 15:46:20.507840    2589 config.go:491] "Configured to push to Venafi" logger="Run" mode="Jetstack Secure API Token" reason="--api-token was specified"
        I0820 15:46:20.508298    2589 loader.go:402] Config loaded from file:  testdata/agent/one-shot/success/kubeconfig.yaml
        I0820 15:46:20.508483    2589 envvar.go:172] "Feature gate default state" feature="InOrderInformers" enabled=true
        I0820 15:46:20.508534    2589 envvar.go:172] "Feature gate default state" feature="WatchListClient" enabled=false
        I0820 15:46:20.508556    2589 envvar.go:172] "Feature gate default state" feature="ClientsAllowCBOR" enabled=false
        I0820 15:46:20.508579    2589 envvar.go:172] "Feature gate default state" feature="ClientsPreferCBOR" enabled=false
        I0820 15:46:20.508612    2589 envvar.go:172] "Feature gate default state" feature="InformerResourceVersion" enabled=false
        I0820 15:46:20.508963    2589 type.go:168] "Request Body" body=""
        I0820 15:46:20.509047    2589 round_trippers.go:527] "Request" curlCommand=<
        	curl -v -XGET  -H "Accept: application/vnd.kubernetes.protobuf,application/json" -H "User-Agent: preflight/v0.0.0 (linux/amd64) kubernetes/$Format" 'https://192.0.2.1:8443/api/v1/namespaces/kube-system'
         >
        
    agent_test.go:60: STDERR
        
    agent_test.go:61: 
        	Error Trace:	/home/runner/work/jetstack-secure/jetstack-secure/cmd/agent_test.go:61
        	Error:      	Received unexpected error:
        	            	signal: killed
        	Test:       	TestAgentRunOneShot
        	Messages:   	context deadline exceeded

[mladen-rusev-cyberark]

tests in cmd are now being run