Support built-in testing infrastructure with qbs build.

I.e. define WITH_TESTS for debug builds or if the respective
environment variable is set.

Change-Id: I2764ece50d0783e8cb030b1695e6f59cf6063248
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>

src/plugins/cpptools/cpptools.qbs

@@ -1,6 +1,7 @@
 import qbs.base 1.0
 
 import "../QtcPlugin.qbs" as QtcPlugin
+import "../../../qbs/defaults.js" as Defaults
 
 QtcPlugin {
     name: "CppTools"
@@ -107,6 +108,11 @@ QtcPlugin {
         "TypeHierarchyBuilder.h"
     ]
 
+    Group {
+        condition: Defaults.testsEnabled(qbs)
+        files: ["cppcodegen_test.cpp", "cppcompletion_test.cpp"]
+    }
+
     ProductModule {
         Depends { name: "CPlusPlus" }
     }