v2: extract test helpers to their own module (test-support?)

[boneskull]

Currently, there are some "test helpers" in @appium/base-driver and @appium/base-plugin which are intended for use by extension authors.

In their current state, they are both awkward to reference e.g., require('@appium/base-driver/build/test/helpers'), lack type information, and make builds more complex (because we do not compile and publish anything in test otherwise).

~~test-support is seemingly orphaned--no packages depend on either @appium/test-support or appium-test-support. Unless there's a private project somewhere (and even then, this would be a major bump), I think it's fair game to repurpose since the proposed changes here are in line with the original intent of that package.~~

UPDATE test-support is not orphaned; npmjs.com does not show devdependencies (I forgot). So any changes here will need to be backwards compat, or we create a new package. There's ...some...useful stuff in there.

Further, some test helpers in elsewhere (like in appium) are useful in other contexts as well, and do things like "install plugin x". We can move those over as well.

[jlipps]

👍 great idea