tests/functional/lang: Change base path to the lang directory

Currently, all tests are relative to `./tests/functional` instead of
`./tests/functional/lang`. Whether this is a historical artefact or as
intended, the current move is to align the tests with the new design of
functional2, preparing them for an easier migration.

Change-Id: Ie394691b071488a8000a005080b9167786d5bd9a

tests/functional/lang/eval-fail-flake-ref-to-string-negative-integer.err.exp

@@ -1,12 +1,12 @@
 error:
        … while calling the 'seq' builtin
-         at /pwd/lang/eval-fail-flake-ref-to-string-negative-integer.nix:1:16:
+         at /pwd/eval-fail-flake-ref-to-string-negative-integer.nix:1:16:
             1| let n = -1; in builtins.seq n (builtins.flakeRefToString {
              |                ^
             2|   type  = "github";
 
        … while calling the 'flakeRefToString' builtin
-         at /pwd/lang/eval-fail-flake-ref-to-string-negative-integer.nix:1:32:
+         at /pwd/eval-fail-flake-ref-to-string-negative-integer.nix:1:32:
             1| let n = -1; in builtins.seq n (builtins.flakeRefToString {
              |                                ^
             2|   type  = "github";