Previously the (non-advanced) playground was only correctly loadging
on localhost because you can put an arbitrary number of "../"s in front
of a relative URL and it just takes you to the root directory.
Since prepare.js and blockly.mjs are going to be needed for running
all tests in uncompiled mode (not just the playgrounds), move them
tests/. Further, rename prepare.js to bootstrap.js to better reflect
its purpose.
