selftests: Fix build when $(O) points to a relative path

Build of bpf and tc-testing selftests fails when the relative path of
the build directory is specified.

make -C tools/testing/selftests O=build0
make[1]: Entering directory '/linux_mainline/tools/testing/selftests/bpf'
../../../scripts/Makefile.include:4: *** O=build0 does not exist.  Stop.
make[1]: Entering directory '/linux_mainline/tools/testing/selftests/tc-testing'
../../../scripts/Makefile.include:4: *** O=build0 does not exist.  Stop.

Makefiles of bpf and tc-testing include scripts/Makefile.include file.
This file has sanity checking inside it which checks the output path.
The output path is not relative to the bpf or tc-testing. The sanity
check fails. Expand the output path to get rid of this error. The fix is
the same as mentioned in commit 150a273 ("bpf, preload: Fix build
when $(O) points to a relative path").

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>

Author: musamaanjum

tools/testing/selftests/Makefile

@@ -165,14 +165,16 @@ all: khdr
 		BUILD_TARGET=$$BUILD/$$TARGET;			\
 		mkdir $$BUILD_TARGET  -p;			\
 		$(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET	\
+				O=$(abs_objtree)		\
 				$(if $(FORCE_TARGETS),|| exit);	\
 		ret=$$((ret * $$?));				\
 	done; exit $$ret;
 
 run_tests: all
 	@for TARGET in $(TARGETS); do \
 		BUILD_TARGET=$$BUILD/$$TARGET;	\
-		$(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET run_tests;\
+		$(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET run_tests \
+				O=$(abs_objtree);		    \
 	done;
 
 hotplug:
@@ -223,6 +225,7 @@ ifdef INSTALL_PATH
 	for TARGET in $(TARGETS); do \
 		BUILD_TARGET=$$BUILD/$$TARGET;	\
 		$(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET INSTALL_PATH=$(INSTALL_PATH)/$$TARGET install \
+				O=$(abs_objtree)		\
 				$(if $(FORCE_TARGETS),|| exit);	\
 		ret=$$((ret * $$?));		\
 	done; exit $$ret;