Add test for downstreamPlaceholder

This is good in general, but in particular ensures when we heavily
refactor it in the next commit there is less likelihood for an
unintentional change in behavior to sneak in.

src/libstore/tests/downstream-placeholder.cc

@@ -0,0 +1,16 @@
+#include <gtest/gtest.h>
+
+#include "derivations.hh"
+
+namespace nix {
+
+TEST(Derivation, downstreamPlaceholder) {
+    ASSERT_EQ(
+        downstreamPlaceholder(
+            (const Store &)*(const Store *)nullptr, // argument is unused
+            StorePath { "g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-foo.drv" },
+            "out"),
+        "/0c6rn30q4frawknapgwq386zq358m8r6msvywcvc89n6m5p2dgbz");
+}
+
+}