tests: shell: auto-run kmemleak if its available

tests: shell: auto-run kmemleak if its available

On my test vm a full scan takes almost 5s. As this would slowdown
the test runs too much, only run them every couple of tests.

This allows to detect when there is a leak reported at the
end of the script, and it allows to narrow down the test
case/group that triggers the issue.

Add new -K flag to force kmemleak runs after each test if its
available, this can then be used to find the exact test case.

Signed-off-by: Florian Westphal <fw@strlen.de>