add test for mimalloc override on Windows (issue #981)

test/main-override-dep.cpp

@@ -0,0 +1,15 @@
+// Issue #981: test overriding allocation in a DLL that is compiled independent of mimalloc. 
+// This is imported by the `mimalloc-test-override` project.
+#include <string>
+#include "main-override-dep.h"
+
+std::string TestAllocInDll::GetString()
+{
+	char* test = new char[128];
+	memset(test, 0, 128);
+	const char* t = "test";
+	memcpy(test, t, 4);
+	std::string r = test;
+	delete[] test;
+	return r;
+}