fix(3p/nix/tests): Fail DummyStore::addToStore on non-existent srcPath

Unfortunately, to guarantee correct behaviour of some evaluation (!)
tests, addToStore needs to actually check whether passed in source
files exist and fail appropriately.

There is a chance that the dependency on this behaviour is actually a
bug in the upstream test suite, but my attempts at finding out more
about this from, say, the git history have so far been unsuccessful.

Change-Id: I311999ea28fcedf5da13a4e627b1c1c8e4e59cbd
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1276
Reviewed-by: isomer <isomer@tvl.fyi>
Reviewed-by: glittershark <grfn@gws.fyi>
Tested-by: BuildkiteCI

third_party/nix/src/tests/dummy-store.hh

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <filesystem>
 #include "libstore/store-api.hh"
 
 namespace nix::tests {
@@ -20,7 +21,11 @@ class DummyStore final : public Store {
                   bool recursive = true, HashType hashAlgo = htSHA256,
                   PathFilter& filter = defaultPathFilter,
                   RepairFlag repair = NoRepair) {
-    return "/nix/store/g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x";
+    if (srcPath == "/exists-for-tests") {
+      return "/nix/store/g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x";
+    }
+
+    throw SysError("file does not exist");
   }
 
   Path addTextToStore(const std::string& name, const std::string& s,