Further optimise instance test data generation

Following on the lessons from this week (commit 7806125e), this patch
replaces generation of instances `suchThat` they are smaller than on
node with direct generation of instances with upper bounds on
memory/disk/cpu.

For a particular case I was investigating, this reduces the overall
duration of tests by almost 50%. I don't expect it to speed up this
nice in most cases, but it's too good an optimisation to pass up.

The patch also does a trivial simplification (if … then True else
False, d'oh).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>