message: build: eliminate warnings with Xcode 10.3
diff: namespace opencv_test { namespace {
  +#if defined __clang__ && defined __APPLE__
  +#pragma clang diagnostic ignored "-Wself-assign-overloaded" // explicitly assigning value of variable of type '...' to itself (p1 = p1)
  +#endif
  +
  #ifdef GTEST_CAN_COMPARE_NULL
  # define EXPECT_NULL(ptr) EXPECT_EQ(NULL, ptr)
  #else
  