Allow overriding the target test count

This is not perfect, as the override applies identical target test
values to both the 'fast' and 'slow' tests (making the slow tests take
a long time), but it allows a quick override for manual runs.

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