Build tests subdirectory

Requires BUILD_TESTS qmake variable to be set

Change-Id: Ida266ce96ec4725608e1290908c546b32ba681ac
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

qtcreator.pro

@@ -13,6 +13,7 @@ CONFIG   += ordered
 
 SUBDIRS = src share lib/qtcreator/qtcomponents
 unix:!macx:!isEmpty(copydata):SUBDIRS += bin
+!isEmpty(BUILD_TESTS):SUBDIRS += tests
 
 OTHER_FILES += dist/copyright_template.txt \
     $$files(dist/changes-*) \