Update test scripts to allow watch usage

[marvinhagemeister]

When working on preact-render-to-string I find myself manually building the test command for watch usage. This PR rewrites our test scripts so that we can pass --watch to both test:mocha and test:mocha:compat to enable watch mode.

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[rschristian]

Ah good call.

Can't remember why I wrote it that way originally. Didn't want to switch between the two? Dunno.