selftests: sync: use TEST_CUSTOM_PROGS instead of TEST_PROGS

lib.mk var TEST_CUSTOM_PROGS is for tests that need custom build
rules. TEST_PROGS is used for test shell scripts. Fix it to use
TEST_CUSTOM_PROGS. lib.mk will run and install them.
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/sync/Makefile

@@ -2,9 +2,11 @@ CFLAGS += -O2 -g -std=gnu89 -pthread -Wall -Wextra
 CFLAGS += -I../../../../usr/include/
 LDFLAGS += -pthread
 
-TEST_PROGS = sync_test
+# lib.mk TEST_CUSTOM_PROGS var is for custome tests that need special
+# build rules. lib.mk will run and install them.
+TEST_CUSTOM_PROGS = sync_test
 
-all: $(TEST_PROGS)
+all: $(TEST_CUSTOM_PROGS)
 
 include ../lib.mk