Fix end to end tests and some other improvements

[YashdalfTheGray]

Description of changes:

• added a log collector workflow that will pull failed CNI plugin tests log and attach them as artifacts
• added a TESTING.md file that goes over some of the testing setup
• migrated ioutil to os calls
• changed all of the tests to have unique network namespaces
• updated vpc-tunnel test to pass the right arguments to exec
• added the right required setup (creating eth1 and eth2 dummy links) to the github workflow
• use a different set of inputs for the TestAddDelBlockIMDS test and the TestAddDel test
• added a template builder and fields map to make the vpc-branch-eni tests read a little better

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ubhattacharjya]

Can we just have validateAfterAddCommon and remove the above validateAfterAdd ?

[ofiliz]

Thanks for these fixes and other improvements.

We should consider renaming "e2e-tests" to "func-test" or whatever ecs-agent uses to use the same terminology and possibly even the same Make targets. But that can be addressed later in a separate PR.