add unit test for trainer sdk

[briangallagher]

What this PR does / why we need it:
Add some initial unit tests for trainer_client.py

Which issue(s) this PR fixes (optional, in Fixes #<issue number>, #<issue number>, ... format, will close the issue(s) when PR gets merged):

Fixes #16

Checklist:

• Docs included if any changes are user facing

[eoinfennessy]

Just a very quick review, but this looks great.

I will have a proper look tomorrow.

[kramaranya]

We create new mock objects every test run, for example get_mock_pod_list and create_cluster_training_runtime. Can we maybe create them once per session and reuse them?
Other than that looks great! Thank you!

[tenzen-y]

Thank you

/approve
Leaving /lgtm on @kramaranya @Electronic-Waste

[andreyvelich]

Thank you for this @briangallagher!
let me take a look at this early next week.
/hold
/assign @andreyvelich

[andreyvelich]

Thanks for the update @briangallagher!
Overall, lgtm, just small comments.

[andreyvelich]

/ok-to-test

[andreyvelich]

Thanks for this amazing contribution @briangallagher!
/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: andreyvelich, tenzen-y

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [andreyvelich]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[andreyvelich]

/hold cancel