selftests: Extract run_kselftest.sh and generate stand-alone test list

Instead of building a script on the fly (which just repeats the same
thing for each test collection), move the script out of the Makefile and
into run_kselftest.sh, which reads kselftest-list.txt.

Adjust the emit_tests target to report each test on a separate line so
that test running tools (e.g. LAVA) can easily remove individual
tests (for example, as seen in [1]).

[1] https://github.com/Linaro/test-definitions/pull/208/commits/2e7b62155e4998e54ac0587704932484d4ff84c8



Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>