feat: Add test coverage for cofidectl-debug-container

[nialdaly]

Summary

• This PR adds test coverage for cofidectl-debug-container and further decomposes main.go.
• These changes have been tested locally (using cofidectl) and work as expected:

just integration-test single-trust-zone

...
2025/01/20 18:19:15 INFO ...pong
+ return 0
+ show_workload_status
++ kubectl get pods -l app=ping-pong-client -n demo -o 'jsonpath={.items[0].metadata.name}' --context kind-kind
+ POD_NAME=ping-pong-client-5f7cc97857-9nxtm
++ ./cofidectl workload status --namespace demo --pod-name ping-pong-client-5f7cc97857-9nxtm --trust-zone tz1
+ WORKLOAD_STATUS_RESPONSE='✅ Complete: Successfully executed emphemeral debug container in ping-pong-client-5f7cc97857-9nxtm

Trust bundles received
* spiffe://td1
    Certificate "17:26:19:31:D9:90:74:18:C1:D5:33:01:9E:3A:90:84:A0:1D:DF:45:D5:C1:8B:BC:86:F3:77:9E:92:AC:99:B3"
    is a CA certificate
    Valid from "2025-01-20T18:18:29Z" to "2025-01-21T06:18:39Z"
    Subject: SERIALNUMBER=318461288371843035447025893851366322856,CN=cofide.io,O=Cofide,C=UK
    URIs: spiffe://td1
    Signature algorithm: SHA256-RSA
    Issuer: SERIALNUMBER=318461288371843035447025893851366322856,CN=cofide.io,O=Cofide,C=UK

SVIDs received
* spiffe://td1/ns/demo/sa/ping-pong-client
    Certificate "1C:6A:EC:0E:24:86:AD:B4:51:F2:57:D8:C8:ED:95:4A:55:C9:F4:DA:40:36:C1:33:5C:D4:41:76:4E:FB:C9:C9"
    Valid from "2025-01-20T18:18:58Z" to "2025-01-20T22:19:08Z"
    Subject: O=SPIRE,C=US
    URIs: spiffe://td1/ns/demo/sa/ping-pong-client
    Signature algorithm: SHA256-RSA
    Issuer: SERIALNUMBER=318461288371843035447025893851366322856,CN=cofide.io,O=Cofide,C=UK
    SVID verified against trust bundle'
+ [[ ✅ Complete: Successfully executed emphemeral debug container in ping-pong-client-5f7cc97857-9nxtm

Trust bundles received
* spiffe://td1
    Certificate "17:26:19:31:D9:90:74:18:C1:D5:33:01:9E:3A:90:84:A0:1D:DF:45:D5:C1:8B:BC:86:F3:77:9E:92:AC:99:B3"
    is a CA certificate
    Valid from "2025-01-20T18:18:29Z" to "2025-01-21T06:18:39Z"
    Subject: SERIALNUMBER=318461288371843035447025893851366322856,CN=cofide.io,O=Cofide,C=UK
    URIs: spiffe://td1
    Signature algorithm: SHA256-RSA
    Issuer: SERIALNUMBER=318461288371843035447025893851366322856,CN=cofide.io,O=Cofide,C=UK

SVIDs received
* spiffe://td1/ns/demo/sa/ping-pong-client
    Certificate "1C:6A:EC:0E:24:86:AD:B4:51:F2:57:D8:C8:ED:95:4A:55:C9:F4:DA:40:36:C1:33:5C:D4:41:76:4E:FB:C9:C9"
    Valid from "2025-01-20T18:18:58Z" to "2025-01-20T22:19:08Z"
    Subject: O=SPIRE,C=US
    URIs: spiffe://td1/ns/demo/sa/ping-pong-client
    Signature algorithm: SHA256-RSA
    Issuer: SERIALNUMBER=318461288371843035447025893851366322856,CN=cofide.io,O=Cofide,C=UK
    SVID verified against trust bundle != *\S\V\I\D\ \v\e\r\i\f\i\e\d\ \a\g\a\i\n\s\t\ \t\r\u\s\t\ \b\u\n\d\l\e* ]]
+ echo 'cofidectl workload status successful'
cofidectl workload status successful
+ down
+ ./cofidectl down
✅ Uninstalled: Uninstallation completed for kind in tz1

+ echo 'Success!'
Success!

[markgoddard]

No need to change, but another way to do this is to make the code under test more amenable to testing, i.e. decouple from IO. e.g. https://dev.to/lucassha/unit-testing-stdout-in-go-1jd