selftests: find echo binary to use -ne options

Find the actual echo binary using $(which echo) and use it for
formatted output with -ne. On some systems, the default echo command
doesn't handle the -e option and the output looks like this (arm64
build):

-ne Emit Tests for alsa

-ne Emit Tests for amd-pstate

-ne Emit Tests for arm64

This is for example the case with the KernelCI Docker images
e.g. kernelci/gcc-10:x86-kselftest-kernelci. With the actual echo
binary (e.g. in /bin/echo), the output is formatted as expected (x86
build this time):

Emit Tests for alsa
Emit Tests for amd-pstate
Skipping non-existent dir: arm64

Only the install target is using "echo -ne" so keep the $ECHO variable
local to it.

Reported-by: "kernelci.org bot" <[e-mail]>
Fixes: 3297a4df805d ("kselftests: Enable the echo command to print newlines in Makefile")
Signed-off-by: Guillaume Tucker <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/Makefile

···
 	@# While building kselftest-list.text skip also non-existent TARGET dirs:
 	@# they could be the result of a build failure and should NOT be
 	@# included in the generated runlist.
+	ECHO=`which echo`; \
 	for TARGET in $(TARGETS); do \
 		BUILD_TARGET=$$BUILD/$$TARGET;	\
-		[ ! -d $(INSTALL_PATH)/$$TARGET ] && echo "Skipping non-existent dir: $$TARGET" && continue; \
-		echo -ne "Emit Tests for $$TARGET\n"; \
+		[ ! -d $(INSTALL_PATH)/$$TARGET ] && $$ECHO "Skipping non-existent dir: $$TARGET" && continue; \
+		$$ECHO -ne "Emit Tests for $$TARGET\n"; \
 		$(MAKE) -s --no-print-directory OUTPUT=$$BUILD_TARGET COLLECTION=$$TARGET \
 			-C $$TARGET emit_tests >> $(TEST_LIST); \
 	done;