Remove $$PREBID_GLOBAL$$ from unit tests

[dbemiller]

The $$PREBID_GLOBAL$$ references are causing a few problems in our test code.

First, they're fragile. #1437 was logged about this, and in the time it took to fix the existing references, several more PRs got merged which broke them again.

Second, they cause weird behavior. #1427 surfaced an issue where a test passed when using config.setConfig, but failed when using $$PREBID_GLOBAL$$.setConfig--even though they're the same function. I traced the root problem to the use of window. Currently, our webpack preprocessor is making a new bundle of Prebid for every unit test file... but the window object (and therefore $$PREBID_GLOBAL$$) persists across files. This is a very subtle bug which makes writing good unit tests tricky.

Lastly, they prevent us from improving our unit test performance. Due to the window issue, many tests surrounding $$PREBID_GLOBAL$$ fail if we do change the way we bundle the code for testing. It's unfortunate, because the unit tests would only take ~15s to complete (rather than the ~45s they take currently). This difference will only increase as we add new unit test files.

We should refactor our code to be more testable, and remove all $$PREBID_GLOBAL$$ references from the existing tests.

[dbemiller]

Fixing this will be a marathon, not a sprint.

My proposal for working towards a solution is outlined in #1510. By encapsulating $$PREBID_GLOBAL$$ state into smaller objects, and injecting those objects as dependencies throughout most of the code, we will gradually work towards a codebase which does not rely on $$PREBID_GLOBAL$$ for testing.

[stale]

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