selftests: Fix installation for splice test

Simplify the Makefile rules so that the test is
automatically installed (and cleaned) by leveraging
the TEST_GEN_PROGS_EXTENDED definition.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>