selftests: use printf instead of echo -ne

Rather than trying to guess which implementation of "echo" to run with
support for "-ne" options, use "printf" instead of "echo -ne".  It
handles escape characters as a standard feature and it is widespread
among modern shells.

Reported-by: "kernelci.org bot" <bot@kernelci.org>
Suggested-by: David Laight <David.Laight@ACULAB.COM>
Fixes: 3297a4df

 ("kselftests: Enable the echo command to print newlines in Makefile")
Fixes: 79c16b1120fe ("selftests: find echo binary to use -ne options")
Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>