Using #!/usr/bin/env bash instead of #!/bin/bash in the BASH scripts of the e2e tests

Appears that on Mac the default /bin/bash shell is not the newest version. For some of the test cases we use BASH features which are available in the latest versions.

Currently, the bash scripts use

#!/bin/bash

at the beginning, which works on recent Linux distros, but not on Macs. Mac users need to install a recent BASH version with macports or homebrew. However, that bash version won't replace the default one. In order to keep the shell scripts consistent, we should use a different shebang line, like this:

#!/usr/bin/env bash

See: https://stackoverflow.com/questions/10376206/what-is-the-preferred-bash-shebang