Add modifycontroller SyncPVC unit tests

[AndrewSirenko]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug

/kind cleanup

/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:

Add modifycontroller SyncPVC unit tests. Including a retro-active unit test for #422.

Also de-duplicate/document some of the unit test boilerplate to help new contributors understand the code.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

/hold

There are a couple of questions in PR marked TODO Q.

Does this PR introduce a user-facing change?:

NONE

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[AndrewSirenko]

This seems like a code smell.

Happy to attempt to try to refactor resizer's testutils to implement Builder pattern to prevent by allowing developers to use MakePVC().WithXXX().WithYYY() style in the test-cases themselves, similar to what k/k has.

This work was started in #402, but was pushed back to once all-in-one sidecar project is here.

[AndrewSirenko]

Note that I have set resync period to 0 because shared_informer does not support resyncing:

W1023 19:04:49.983836 13491 shared_informer.go:569] The specified resyncPeriod 1s is invalid because this shared informer doesn't support resyncing

This also let us cut out any sleeps in unit tests (instant tests!)