ENH: Add test to see if INSTALL_FILES actually worked.

Tests/SimpleInstall/inst.cxx

@@ -3,6 +3,7 @@
 #ifdef STAGE_2
 # include <foo/lib1.h>
 # include <foo/lib2renamed.h>
+# include <lib3.h>
 #else
 # include "lib1.h"
 # include "lib2.h"

Tests/SimpleInstallS2/inst.cxx

@@ -3,6 +3,7 @@
 #ifdef STAGE_2
 # include <foo/lib1.h>
 # include <foo/lib2renamed.h>
+# include <lib3.h>
 #else
 # include "lib1.h"
 # include "lib2.h"