tests:Disable maintenance1_ext by default

Tests on some devices are incorrectly attempting to enable maintenance1
extension. Make sure it's set to off by default.
diff --git a/tests/layer_validation_tests.cpp b/tests/layer_validation_tests.cpp
index 98b9c59..9ef889d 100644
--- a/tests/layer_validation_tests.cpp
+++ b/tests/layer_validation_tests.cpp
@@ -355,7 +355,7 @@
    protected:
     ErrorMonitor *m_errorMonitor;
     bool m_enableWSI;
-    bool m_enable_maintenance1_ext;
+    bool m_enable_maintenance1_ext = false;
 
     virtual void SetUp() {
         std::vector<const char *> instance_layer_names;