Fix language tests a bit

- Remove some stray saved error messages that didn't correspond to any
  test, because they were renamed in
  d11faa0.

- 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

tests/functional/lang/eval-fail-bad-string-interpolation-2.err.exp

@@ -1 +1 @@
-error: operation 'addToStoreFromDump' is not supported by store 'dummy'
+error: getting status of '/pwd/lang/fnord': No such file or directory

tests/functional/lang/eval-fail-dup-dynamic-attrs.err.exp

@@ -1,4 +1,14 @@
-error: dynamic attribute 'b' already defined at /pwd/lang/eval-fail-dup-dynamic-attrs.nix:2:11
+error:
+ … while evaluating the attribute 'set'
+
+ at /pwd/lang/eval-fail-dup-dynamic-attrs.nix:2:3:
+
+ 1| {
+ 2| set = { "${"" + "b"}" = 1; };
+ | ^
+ 3| set = { "${"b" + ""}" = 2; };
+
+ error: dynamic attribute 'b' already defined at /pwd/lang/eval-fail-dup-dynamic-attrs.nix:2:11
 
  at /pwd/lang/eval-fail-dup-dynamic-attrs.nix:3:11:
 

tests/functional/lang/eval-fail-nonexist-path.err.exp

@@ -1 +1 @@
-error: operation 'addToStoreFromDump' is not supported by store 'dummy'
+error: getting status of '/pwd/lang/fnord': No such file or directory