Hack around wall clock time going backwards.

As reported in https://github.com/jmmv/kyua/issues/155, the wall clock
time can go backwards resulting in an apparent negative delta. As
a workaround, convert such deltas to 1us. This allows tests to run
successfully in MIPS64 qemu.

Approved by: jmmv (maintainer)
Sponsored by: DARPA, AFRL