selftests: Extract single-test shell logic from lib.mk

In order to improve the reusability of the kselftest test running logic,
this extracts the single-test logic from lib.mk into kselftest/runner.sh
which lib.mk can call directly. No changes in output.

As part of the change, this moves the "summary" Makefile logic around
to set a new "logfile" output. This will be used again in the future
"emit_tests" target as well.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>