Enable test building using environment variable

Makes it easy to globally opt-in to testable Creator builds,
by avoiding the need to remember adding BUILD_TEST=x each
time one runs qmake in a fresh Creator build.

Change-Id: Ic1ea9efd05dbdac1e402f245b5320ee5ff33ae91
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

qtcreator.pri

@@ -59,6 +59,9 @@ defineReplace(stripSrcDir) {
     return($$relative_path($$absolute_path($$1, $$OUT_PWD), $$_PRO_FILE_PWD_))
 }
 
+QTC_BUILD_TESTS = $$(QTC_BUILD_TESTS)
+!isEmpty(QTC_BUILD_TESTS):TEST = $$QTC_BUILD_TESTS
+
 !isEmpty(BUILD_TESTS):TEST = 1
 
 isEmpty(TEST):CONFIG(debug, debug|release) {