Previously, it was necessary to support older versions of Python for
compatibility. F-strings were released in Python 3.6. Python 3.5 was
marked as unsupported by the end of 2020, so now it's possible to start
using f-strings safely for better readability and performance.
The mount points are unmounted in the main process after it
detects the app process died. By testing the `tmpfs: true`
first, it happens that main could start the `tmpfs: false`
test case before main cleans the old process mount points.
- The mode of testdir was changed to allow reading from other users/groups.
- The java multipart test now uploads the file into an app writable dir.
- The build directory was made readable for other users.
- The python environment test now uses the HOME env var instead of PWD
because the latter is not set by the root shell (/bin/sh) by default.
- The node `node_modules` directory now is copied into the `testdir` instead
of using symlinks.
