The non-git case was not well documented and not tested, and it was broken since we switched the build infrastructure.
In the git case, an update is to do a git pull. In the non-git case, it downloads a zip and unpacks it. The old build infra apparently had builds created for this; instead, I made it download it directly from github. Perhaps we should consider creating builds for this as well eventually?
Also add a test for this, so we never break it again.
Also fix the can_be_installed which was always broken it seems - it returns True for success or a string for error, but didn't check if the output is True.
Includes a test, which hacks up emsdk to make it think it's on 32-bit, and verifies the error and message.
Keep the basic workflow test (checking what a user would do to get started) in bash, but otherwise it's more convenient for most tests to be in python.
