Add a bunch more tweaks to the parallel test compilation

- put all the precompiled test binaries in $DEST so they show up in bundles and can be re-run individually afterwards
- support cases where parallel is not installed (when using dyntest-unit, for example, this is much more common, since it's designed to be run outside the Dockerfile)
- use "mktemp -d" instead of "/tmp" directly for our temporary parallel HOME
- update the default PARALLEL_JOBS to be the value of "nproc" instead of 0, since "0 means as many as possible" (see https://www.gnu.org/software/parallel/man.html#jobs_n)