runtests: spawn a new process for the test runner

runtests: spawn a new process for the test runner

When the -j option is given, a new process is spawned in which the test
programs are run and from which test servers are started. Only one
process can be started at once, but this is sufficient to test that the
infrastructure can isolate those functions in a new task. There should
be no visible difference between the two modes at the moment.

Ref: #10818
Closes #11064