[kn-admin] Add Unit Test and e2e test

[zhanggbj]

For kn-admin plugin, use go test framework to fill up Unit Test and e2e test.

[zhanggbj]

UT merged with #14 by @lanceliuu
Xu, any plan for e2e?

[xliuxu]

Progress for adding unit test & e2e test:
The unit tests part is done in PR #31

?       knative.dev/client-contrib/plugins/admin/cmd    [no test files]
ok      knative.dev/client-contrib/plugins/admin/core   4.482s  coverage: 50.0% of statements
?       knative.dev/client-contrib/plugins/admin/pkg    [no test files]
ok      knative.dev/client-contrib/plugins/admin/pkg/command    1.203s  coverage: 100.0% of statements
ok      knative.dev/client-contrib/plugins/admin/pkg/command/domain     2.188s  coverage: 97.7% of statements
ok      knative.dev/client-contrib/plugins/admin/pkg/command/registry   4.872s  coverage: 94.9% of statements
?       knative.dev/client-contrib/plugins/admin/pkg/testutil   [no test files]

I will continue to add e2e tests in another pull request.

[zhanggbj]

e2e which need to fill up
Autoscaling

• autoscaling update --scale-to-zero @zhanggbj
• autoscaling update --no-scale-to-zero @zhanggbj

Domain

• domain set @lanceliuu
• domain set with selector @lanceliuu
• domain unset @lanceliuu

Registry

• registry add @12345lcr
• registry remove @12345lcr