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>
diff --git a/Makefile b/Makefile
index 4f054bc..4321ed0 100644
--- a/Makefile
+++ b/Makefile
@@ -107,7 +107,7 @@
 .PHONY: msg_start
 msg_start:
 	@echo "Building ${PLAT}"
-	@echo "Using the following tests file: ${TESTS_FILE}"
+	@echo "Selected set of tests: ${TESTS}"
 
 # Include test images makefiles.
 include tftf/framework/framework.mk
diff --git a/tftf/tests/tests.mk b/tftf/tests/tests.mk
index 75c409b..f5414e2 100644
--- a/tftf/tests/tests.mk
+++ b/tftf/tests/tests.mk
@@ -6,6 +6,14 @@
 
 TESTS ?= standard
 
+tests_files	:= $(wildcard tftf/tests/*.xml)
+tests_sets	:= $(patsubst tftf/tests/tests-%.xml,%,${tests_files})
+
+PHONY: help_tests
+help_tests:
+	@echo "Available sets of tests:"
+	@$(foreach t, ${tests_sets}, printf " - %s\n" ${t};)
+
 TESTS_FILE	:= tftf/tests/tests-${TESTS}.xml
 TESTS_MAKEFILE	:= tftf/tests/tests-${TESTS}.mk