Fixup the unit.client_test.LocalClientTestCase.test_cmd_subset from #35720

[rallytime]

What does this PR do?

The bug fix for #20575, which is located in PR #35720, caused a test failure in the unit.client_test tests that wasn't caught until later. This PR fixes that test.

Background information on why I am fixing it this way, after confirming with @thatch45 :

• The ret option in this function actually stands for returner and should just be a pass-through and not overwritten in the cmd_subset function as was originally occurring in the function before the fix in fix 20575, make subset really return random subset #35720. Therefore, the expected return in this unit test should be ret=''.
• I am using a try/except on each of these tests, because now that the cmd_subset function actually returns a random subset of minions, sometimes the sub=1 function operates on minion2 and sometimes it is minion1. The same goes for the list of minions for sub=10. Since this assert is against a list, sometimes the list of minions will be called in differing orders.

What issues does this PR fix or reference?

Refs #35720

Previous Behavior

Test failing on 2016.3 branch.

New Behavior

Test will pass now.

Tests written?

Yes

Please review Salt's Contributing Guide for best practices.

[hu-dabao]

@rallytime
Good job, thanks for fixing the tests. 👍

[rallytime]

@hu-dabao No problem! Thanks for fixing the code. :D