selftests: remove duplicated all and clean target

Currently, kselftest use TEST_PROGS, TEST_PROGS_EXTENDED, TEST_FILES to
indicate the test program, extended test program and test files. It is
easy to understand the purpose of these files. But mix of compiled and
uncompiled files lead to duplicated "all" and "clean" targets.

In order to remove the duplicated targets, introduce TEST_GEN_PROGS,
TEST_GEN_PROGS_EXTENDED, TEST_GEN_FILES to indicate the compiled
objects.

Also, the later patch will make use of TEST_GEN_XXX to redirect these
files to output directory indicated by KBUILD_OUTPUT or O.

And add this changes to "Contributing new tests(details)" of
Documentation/kselftest.txt.

Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>