Fix language tests a bit

- Remove some stray saved error messages that didn't correspond to any
  test, because they were renamed in
  d11faa01b5.

- Need `--eval` in test failure test in order to get in "read-only" mode
  where we don't try to write to the store. (The other tests already do
  this.)

- Need `--strict` so top-level attribute sets are still forced, like
  they are without `--eval`.

tests/functional/lang.sh

@@ -68,7 +68,7 @@ for i in lang/eval-fail-*.nix; do
     echo "evaluating $i (should fail)";
     i=$(basename "$i" .nix)
     if
-        expectStderr 1 nix-instantiate --show-trace "lang/$i.nix" \
+        expectStderr 1 nix-instantiate --eval --strict --show-trace "lang/$i.nix" \
             | sed "s!$(pwd)!/pwd!g" > "lang/$i.err"
     then
         diffAndAccept "$i" err err.exp