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.
