The tests are defined by the script 'test/runtests.jl' from packages and the
parallelism depends on the implementation of this script. Therefore, 'julia'
is launched using local worker processes accordingly with 'parallel?'.
* guix/build/julia-build-system.scm (check): Set the JULIA_CPU_THREADS
environment variable and invoke julia with the '--procs' option.
* guix/build-system/julia.scm (julia-build)[parallel-tests?]: New argument.
Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>