bpf/tests: Allow different number of runs per test case

This patch allows a test cast to specify the number of runs to use. For
compatibility with existing test case definitions, the default value 0
is interpreted as MAX_TESTRUNS.

A reduced number of runs is useful for complex test programs where 1000
runs may take a very long time. Instead of reducing what is tested, one
can instead reduce the number of times the test is run.

Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210914091842.4186267-2-johan.almbladh@anyfinetworks.com