Add a makefile rule to print set of tests

Sample output:

> make help_tests
Available sets of tests:
 - sdei
 - uncontainable
 - extensive
 - arm-state-switch
 - manual
 - single-fault
 - tsp
 - psci-extensive
 - standard
 - cpu-extensions
 - boot-req
 - psci
 - tftf-validation
 - spm
 - el3-power-state
 - performance
 - template
 - fwu
 - runtime-instrumentation

Change-Id: I8ab5ae0cadfe950cb690d0943148b15e2448f5c8
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>