Let's keep the debug logs in the journal, while logging only
testsute-*.sh stdout/stderr to the console (ba7abf7). This should make
the test output log a bit more readable and potentially the tests itself
a bit faster by avoiding console oversaturation.
Also, it should significantly reduce the size of artifacts kept by CIs.
This adds the ability to specify truncate:PATH for StandardOutput= and
StandardError=, similar to the existing append:PATH. The code is mostly
copied from the related append: code. Fixes#8983.