cns: make unit tests less sensitive to speed of computer

A recent build failed because the CNS unit-tests rely on a test taking
no longer than 100 ms to complete.  Under normal circumstances, this
is reasonable; however, the build machine was suffering high IO load
and didn't managed to complete the unit test in the allotted time
window.

While the build machine shouldn't be suffering from this IO-load,
equally, our unit-tests shouldn't fail on slow machines.

This patch doesn't fix the dependency on system time; rather, it
increases the time window to 1 minute.

Target: master
Requires-notes: no
Requires-book: no
Patch: http://rb.dcache.org/r/6432/
Acked-by: Gerd Behrmann