selftests: add quicktest support

Add quicktest support to enable users to choose to run
tests that complete in a short time. Choosing this option
excludes tests that take longer time complete e.g: timers.
User can specify quicktest option from kernel top level or
selftests directory.

Kernel top level directory:
make quicktest=1 kselftest

tools/testing/selftests directory:
make quicktest=1 run_tests

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>