Add changelog entry

Signed-off-by: Ronald Cron <ronald.cron@arm.com>

ChangeLog.d/tests-common-code.txt

@@ -0,0 +1,5 @@
+Changes
+   * The unit tests now rely on header files in tests/include/test and source
+     files in tests/src. When building with make or cmake, the files in
+     tests/src are compiled and the resulting object linked into each test
+     executable.