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>