Tests: Extract TestDataDir to the coreplugin

Change-Id: Ie290c07c07c13134a57f328e9ae876b2af6974db
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/coreplugin/coreplugin.pro

@@ -238,3 +238,8 @@ else:unix {
     }
 }
 OTHER_FILES += editormanager/BinFiles.mimetypes.xml
+
+equals(TEST, 1) {
+    SOURCES += testdatadir.cpp
+    HEADERS += testdatadir.h
+}