tests: rearrange invocation of halsampler to avoid race conditions

see also: https://github.com/machinekit/machinekit/issues/202

this fixes a wide range of runtests errors, from 'overrun detected
- re-running tests' to runtests with truncated output from halsampler


Signed-off-by: Michael Haberler <git@mah.priv.at>
Merge pull request #207 from mhaberler/fixsampler

tests: rearrange invocation of halsampler to avoid race conditions