selftests/resctrl: Make resctrl_tests run using kselftest framework

In kselftest framework, all tests can be build/run at a time,
and a sub test also can be build/run individually. As follows:
$ make kselftest-all TARGETS=resctrl
$ make -C tools/testing/selftests run_tests
$ make -C tools/testing/selftests TARGETS=resctrl run_tests

However, resctrl_tests cannot be run using kselftest framework,
users have to change directory to tools/testing/selftests/resctrl/,
run "make" to build executable file "resctrl_tests",
and run "sudo ./resctrl_tests" to execute the test.

To build/run resctrl_tests using kselftest framework.
Modify tools/testing/selftests/Makefile
and tools/testing/selftests/resctrl/Makefile.

Even after this change, users can still build/run resctrl_tests
without using framework as before.

Reviewed-by: Reinette Chatre <[e-mail]> # resctrl changes
Reviewed-by: Fenghua Yu <[e-mail]>
Signed-off-by: Shaopeng Tan <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/Makefile

···
 TARGETS += pstore
 TARGETS += ptrace
 TARGETS += openat2
+TARGETS += resctrl
 TARGETS += rlimits
 TARGETS += rseq
 TARGETS += rtc

tools/testing/selftests/resctrl/Makefile

···
-CC = $(CROSS_COMPILE)gcc
 CFLAGS = -g -Wall -O2 -D_FORTIFY_SOURCE=2
-SRCS=$(wildcard *.c)
-OBJS=$(SRCS:.c=.o)
+CFLAGS += $(KHDR_INCLUDES)
 
-all: resctrl_tests
+TEST_GEN_PROGS := resctrl_tests
 
-$(OBJS): $(SRCS)
-	$(CC) $(CFLAGS) -c $(SRCS)
+include ../lib.mk
 
-resctrl_tests: $(OBJS)
-	$(CC) $(CFLAGS) -o $@ $^
-
-.PHONY: clean
-
-clean:
-	$(RM) $(OBJS) resctrl_tests
+$(OUTPUT)/resctrl_tests: $(wildcard *.c)