Some cleanups of old stuff

[mweststrate]

Code change checklist

• Added/updated unit tests
• Updated /docs. For new functionality, at least API.md should be updated
• Verified that there is no significant performance drop (yarn mobx test:performance)

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[urugator]

IIRC there were some tests in V4 that were relevant only for proxy: false (don't have an analog in V5) or had different expectations or required a bit different API to implement (object-api), would be nice to preserve these.

We should consolidate/rethink the behavior of resetGlobalState and it's usage in tests. For some time I've been using it to reset configure(...) at the end of a tests without realizing it actually doesn't reset everything. When I found out I went ahead and tried to change it, just to find out this would stop working:

mobx/packages/mobx/__tests__/v4/mobx4.ts

Lines 3 to 8 in 55260aa

	configure({
	useProxies: "never",
	enforceActions: "never"
	})
	export * from "../../src/mobx"

I also found a test that was passing only because a previous test did not revert the configuration to default.
Ideally we should make sure tests can't have side-effects without the need to manually wrap them like so:

try { 
  /* test logic */
} finally { 
  configure({ foo: "You have to know the default for these set of tests" });
}