gpgscm: Limit the number of parallel jobs.

* ffi.c (do_wait_processes): Suppress the timeout error.
* tests.scm (semaphore): New definition.
(test-pool): Only run a bounded number of tests in parallel.
(test::started?): New function.
(run-tests-parallel): Do not report results, do not start the tests.
(run-tests-sequential): Adapt.
(run-tests): Parse the number of parallel jobs.

This change limits the number of tests that are run in parallel. This
way we do not overwhelm the operating systems' scheduler. As a
side-effect, we also get more accurate runtime information, and it
will be easy to implement timeouts on top of this.

Use TESTFLAGS to limit the number of jobs:

$ make check-all TESTFLAGS=--parallel=16

• Signed-off-by: Justus Winter <justus@g10code.com>