Add new C API for working with derivations

Also test the APIs we just added.

src/libstore-tests/main.cc

@@ -0,0 +1,15 @@
+#include <gtest/gtest.h>
+
+#include "nix/store/tests/test-main.hh"
+
+using namespace nix;
+
+int main(int argc, char ** argv)
+{
+    auto res = testMainForBuidingPre(argc, argv);
+    if (res)
+        return res;
+
+    ::testing::InitGoogleTest(&argc, argv);
+    return RUN_ALL_TESTS();
+}