Split tests makefile

Instead of unconditionally pulling all test source files in tests.mk,
introduce small makefiles that pull in only those source files that
are required to test a particular feature. For every tests manifest
(test-*.xml files), create the associated makefile.

The main benefit is to compile only the required test source files
when selecting a subset of the tests. For example, if one selects the
SDEI XML file, the TFTF binary only needs the SDEI test cases.

Change-Id: I281eb50846edea2d238c86a795fde8747f8b420b
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
diff --git a/tftf/tests/tests-performance.mk b/tftf/tests/tests-performance.mk
new file mode 100644
index 0000000..fc082c6
--- /dev/null
+++ b/tftf/tests/tests-performance.mk
@@ -0,0 +1,9 @@
+#
+# Copyright (c) 2018, Arm Limited. All rights reserved.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+#
+
+TESTS_SOURCES	+=	$(addprefix tftf/tests/performance_tests/,	\
+	smc_latencies.c							\
+)