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" <bot@kernelci.org>
Fixes: 3297a4df

 ("kselftests: Enable the echo command to print newlines in Makefile")
Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>