Makefile: add missing help_tests info on help target

Although 'help_tests' target has been present for a long time, it is
not shown on the main 'help' target output. This commits includes
basic 'help_tests' info on the latter, making it visible.

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
Change-Id: Ief510ae7f340207d416412f81e97e30017866b17
1 file changed