Tests/IncludeDirectories: Include system headers via angle brackets

This is typically how projects include them, and cl's `-external:{I,W}`
flags suppress warnings only when included through angle brackets.

Tests/IncludeDirectories/SystemIncludeDirectories/consumer.cpp

 
-#include "config_iface.h"
+#include <config_iface.h>
+
 #include "upstream.h"
 
 int consumer()

Tests/IncludeDirectories/SystemIncludeDirectories/imported_consumer.cpp

 
-#include "systemlib.h"
+#include <systemlib.h>
 
 int main()
 {

Tests/IncludeDirectories/SystemIncludeDirectories/upstream.h

@@ -2,7 +2,7 @@
 #ifndef UPSTREAM_H
 #define UPSTREAM_H
 
-#include "systemlib.h"
+#include <systemlib.h>
 
 #ifdef _WIN32
 __declspec(dllexport)