remote-store test: Break out IFD expression into a separate file

(cherry picked from commit 9cb287657bec5a969d8bb1678d598d9fa820e60b)

tests/ifd.nix

@@ -0,0 +1,10 @@
+with import ./config.nix;
+import (
+  mkDerivation {
+    name = "foo";
+    bla = import ./dependencies.nix {};
+    buildCommand = "
+      echo \\\"hi\\\" > $out
+    ";
+  }
+)

tests/remote-store.sh

@@ -19,18 +19,7 @@ else
 fi
 
 # Test import-from-derivation through the daemon.
-[[ $(nix eval --impure --raw --expr '
-  with import ./config.nix;
-  import (
-    mkDerivation {
-      name = "foo";
-      bla = import ./dependencies.nix {};
-      buildCommand = "
-        echo \\\"hi\\\" > $out
-      ";
-    }
-  )
-') = hi ]]
+[[ $(nix eval --impure --raw --file ./ifd.nix) = hi ]]
 
 storeCleared=1 NIX_REMOTE_=$NIX_REMOTE $SHELL ./user-envs.sh