Disable IntelliSense for all DNN unit tests (#678)

* Problem:    Visual Studio's vcpkgsrv.exe constantly uses a single CPU core,
            apparently never finishing whatever it's trying to do. Moreover,
            this issue prevents some operations like switching from Debug to
            Release (and vice versa) in the IDE. (Your mileage may vary.)
Workaround: Keep manually killing the vcpkgsrv.exe process.
Solution:   Disable IntelliSense for some files. Which files? Unfortunately
            this seems to be a trial-and-error process.

* Disable IntelliSense for the ResNet declarations

* Disable IntelliSense for even more stuff

* Disable IntelliSense for all DNN unit tests

dlib/test/dnn.cpp

@@ -13,6 +13,8 @@
 
 #include "tester.h"
 
+#ifndef __INTELLISENSE__
+
 namespace
 {
 
@@ -2392,4 +2394,5 @@ namespace
     } a;
 }
 
+#endif // __INTELLISENSE__