fixed compile problems when Makefiles are used

modules/core/test/precomp.cpp

@@ -1 +0,0 @@
-#include "precomp.hpp"

modules/core/test/test_arithm.cpp

@@ -1,4 +1,4 @@
-#include "precomp.hpp"
+#include "test_precomp.hpp"
 #include <iostream>
 
 using namespace cv;

modules/core/test/test_main.cpp

@@ -1,2 +1,2 @@
-#include "precomp.hpp"
+#include "test_precomp.hpp"
 #include "opencv2/gtest/gtest_main.hpp"

modules/core/test/test_precomp.cpp

@@ -0,0 +1 @@
+#include "test_precomp.hpp"