ensure a good clock() value for usage and tests

ensure a good clock() value for usage and tests

We use clock() as a very cheap way of getting a "random" value, but the
manpage warns that this could return -1, so we should be dealing with
this. Additionally, e.g. on hurd-i386 the value increases only slowly –
to slow for our fast running tests for randomness hence producing the
same range in both samples, so we introduce a simple busy-wait loop (as
clock is counting processor time used by the program) in the test which
delays the second sample just enough making our randomness a bit more
predictable.