Adapt the futextest Makefiles to use lib.mk macros for RUN_TESTS and
EMIT_TESTS. For now, we reuse the run.sh mechanism provided by
futextest. This doesn't provide the standard selftests: [PASS|FAIL]
format, but the tests provide very similar output already.
This results in the run_kselftest.sh script for futexes including a
single line: ./run.sh