Better tests (+ more hotfixes) - #2292

This improves our test setup and also fixes two issues that were uncovered while doing this.

• This adds the "leak" test to our CI setup which would have uncovered this issue before the PR was merged: Provide the watcher on the plugin context #2261 (comment)
  Note that apparently "weak" only seems to work properly on Travis when installed manually as opposed to via package.json. I would suppose it is a caching issue.
• This also adds a "build:bootstrap" script which uses the result of a previous "build" to build rollup again. On the CI, we now run this script and rollup's whole test-suite again after the normal test scripts have completed as a smoke test whether e.g. tree-shaking has removed some vital part of the logic. As it turned out, this immediately uncovered two bugs that were caused respectively revealed by the new undefined expression/property logic:
  1. When a namespace is passed to a function, all its members need to be reassigned as we lose track of this variable.
  2. The declarations in for-in and for-of statements are realized as variable declarations without initializers. The new logic now faultily assumed these variables were always "undefined".