selftests: net: af_unix: Fix makefile to use TEST_GEN_PROGS

Makefile uses TEST_PROGS instead of TEST_GEN_PROGS to define
executables. TEST_PROGS is for shell scripts that need to be
installed and run by the common lib.mk framework. The common
framework doesn't touch TEST_PROGS when it does build and clean.

As a result "make kselftest-clean" and "make clean" fail to remove
executables. Run and install work because the common framework runs
and installs TEST_PROGS. Build works because the Makefile defines
"all" rule which is unnecessary if TEST_GEN_PROGS is used.

Use TEST_GEN_PROGS so the common framework can handle build/run/
install/clean properly.

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>