If sh is not bash, some builtin command behave slightly differently.
E.g. if sh is provided by busybox, its builtin test command does not check if
the path is a mount point or not, and 'test -w' only checks the access mode of
the inode. So, even if a readonly filesystem is mounted on a directory,
the test command may succeed.
To avoid such confusion, let's unconditionally use bash.
We want to retain *some* of the full paths in order to test more code paths.
But the default should be to use the command name only. This makes the tests
less visually cluttered.
Those are all consumed by our parser, so they all support comments.
I was considering whether they should have a license header at all,
but in the end I decided to add it because those files are often created
by copying parts of real unit files. And if the real ones have a license,
then those might as well. It's easier to add it than to make an exception.
Also fix one case where the presence of a newline was used to generate
an invalid environment assignment.
Tested: with mkosi, which builds the local tree and run ninja tests.
Check the base case, plus erasing the list, listing the same variable
name more than once and when variables are absent from the manager
execution environment.
Confirmed that `sudo ./test-execute` passes and that modifying the test
cases (or the values of the set variables in test-execute.c) is enough
to make the test cases fail.
