do not define TESTS_SOURCE_DIR in core tests

It's currently unused, and we can use QFINDTESTDATA instead.

Change-Id: Id439b8b6ea3c9dbf5299c271d3974c11a44675bc
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>

tests/auto/core/tests.pri

@@ -13,6 +13,3 @@ INCLUDEPATH += $$PWD
 
 QT += testlib network webenginewidgets widgets
 osx: CONFIG -= app_bundle
-
-# This define is used by some tests to look up resources in the source tree
-DEFINES += TESTS_SOURCE_DIR=\\\"$$PWD/\\\"