Let's make this behave more like all the rest of the meson stuff.
This also is the first step to making it a bit more flexible so we
can define integration tests in different ways as will be seen in
the next commits.
Specifying the test number manually is tedious and prone to errors (as
recently proven). Since we have all the necessary data to work out the
test number, let's do it automagically.
As in 2a5fcfae02
and in 3e67e5c992
using /usr/bin/env allows bash to be looked up in PATH
rather than being hard-coded.
As with the previous changes the same arguments apply
- distributions have scripts to rewrite shebangs on installation and
they know what locations to rely on.
- For tests/compilation we should rather rely on the user to have setup
there PATH correctly.
In particular this makes testing from git easier on NixOS where do not provide
/bin/bash to improve compose-ability.
