imgcodecs: disable OpenEXR in runtime for 3rdparty source code

- builtin OpenEXR source code is outdated
- external OpenEXR distributions are allowed

modules/imgcodecs/test/test_grfmt.cpp

@@ -363,6 +363,6 @@ TEST(Imgcodecs, write_parameter_type)
 
 }} // namespace
 
-#ifdef HAVE_OPENEXR
+#if defined(HAVE_OPENEXR) && defined(OPENCV_IMGCODECS_ENABLE_OPENEXR_TESTS)
 #include "test_exr.impl.hpp"
 #endif