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: NNaresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: NDaniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/splice/Makefile

 TEST_PROGS := default_file_splice_read.sh
-EXTRA := default_file_splice_read
-all: $(TEST_PROGS) $(EXTRA)
+TEST_GEN_PROGS_EXTENDED := default_file_splice_read
 
 include ../lib.mk
-
-EXTRA_CLEAN := $(EXTRA)