Disable Go Unit Test Cache in Go 1.12

[sgreene570]

- What I did
In #818, the flag to disable GOCACHE was removed for go 1.12 compatibility. Turns out, running unit tests with the cache enabled is frustrating. For example, if a test asset (and not a test file itself) is updated, re-running make test-unit will pull test results from the test cache. Also, changing some parts of a unit test may not actually clear the entire test cache for that particular test (even though it should).

The new way to disable the go testing cache is to use -count=1 in the go test command.
(see golang/go#24573)

- How to verify it
make test-unit and verify that no test were pulled from the cache (ie. look for (cached))

- Description for the changelog

Disable Go Cache for unit tests in Makefile

[cgwalters]

/approve
Thanks for this. As this says,

There are two hard things in computer science: cache invalidation, naming things, and off-by-one errors.

And this is incorrect caching, the go test framework's caching would need to know external inputs.

Will give this a bit for someone else to lgtm.

[kikisdeliveryservice]

@sgreene570 and I discovered this yesterday while debugging a unit test, so A+ on this PR 😄

/lgtm

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[runcom]

Great catch Steve!