unittest addCleanup calls are called in reverse order, so we need to reverse
the order of the calls as well, otherwise we won't correctly terminate the
subprocess children
No need to repeat the action in every unit test, but move the tests to a
different class to easily allow adding another class with tests with no
such initialization
Avoid using a temporary file for reading the utilities output, so we can
read it as it comes, ignoring the previous one, and avoiding open/closing
the file.
To keep the output printing on cleanup working, adding an utility function
that reads the output and save it for later printing
Avoid repeating the same operation to launch the utilities all the times,
but provide instead a function that allows to start a process and saves its
output without having to handle this in every test.
Simplify the operation when we just want the final output, still reusing
the same code.
Like for the fprintd test, run the tests under valgrind if the `VALGRIND`
environment variable is set, and use the contents of the variable as the
path to the suppression file.
