Delete RUN_TESTS and EMIT_TESTS overrides and use common defines in
lib.mk. Add new run_tests.sh to do the dependency checks the custom
RUN_TESTS did. Common defines work with the run_tests.sh set as the
TEST_PROGS and defining unprivileged-remount-test in TEST_GEN_FILES.

Kselftest framework builds and installs TEST_GEN_FILES and doesn't run
them via RUN_TESTS and include it in EMIT_TESTS. With this change the
new run_tests.sh runs the test after checking dependencies.

This change also adds Skip handling to return kselftest skip code when
test is skipped to clearly identify when the test is skipped instead of
reporting it as failed.

Output with this change:

TAP version 13
selftests: mount: run_tests.sh
========================================
WARN: No /proc/self/uid_map exist, test skipped.
not ok 1..1 selftests: mount: run_tests.sh [SKIP]
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>
Reviewed-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>