kselftests: Enable the echo command to print newlines in Makefile

In the install section of the main Makefile of kselftests, the echo
command is used with -n flag, which disables the printing of new line
due to which the output contains "\n" chars as follows:

  Emit Tests for alsa\nSkipping non-existent dir: arm64
  Emit Tests for breakpoints\nEmit Tests for capabilities\n

This patch fixes the above bug by using the -e flag.

Signed-off-by: Gautam <gautammenghani201@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>