tests/functional2/lang: fix warnings

While the err file should always be empty, we prefer not have (easily
avoidable) warnings in the log, in order for actual warnings to be
spotted more easily.
Additionally this way no additional changes are required in case they
make use of some depreacted features in the future.

Change-Id: Ie078e2a851b2035d839f6b95d4188e475eb96b2d

tests/functional2/lang/builtins.readDir/test_read_dir.py

@@ -23,6 +23,7 @@ from functional2.testlib.fixtures.snapshot import Snapshot
         },
         "in.nix": CopyFile("in.nix"),
         "out.exp": AssetSymlink("eval-okay.out.exp"),
+        "err.exp": AssetSymlink("eval-okay.err.exp"),
     }
 )
 def test_read_dir(files: Path, nix: Nix, snapshot: Callable[[str], Snapshot]):

tests/functional2/lang/builtins.readFileType/test_read_file_type.py

@@ -23,6 +23,7 @@ from functional2.testlib.fixtures.snapshot import Snapshot
         },
         "in.nix": CopyFile("in.nix"),
         "out.exp": AssetSymlink("eval-okay.out.exp"),
+        "err.exp": AssetSymlink("eval-okay.err.exp"),
     }
 )
 def test_read_file_type(files: Path, nix: Nix, snapshot: Callable[[str], Snapshot]):

tests/functional2/lang/search-path/test_search_path.py

@@ -16,6 +16,7 @@ from functional2.testlib.fixtures.snapshot import Snapshot
         "lib.nix": CopyFile("../lib.nix"),
         "in.nix": CopyFile("in.nix"),
         "out.exp": AssetSymlink("eval-okay.out.exp"),
+        "err.exp": AssetSymlink("eval-okay.err.exp"),
     }
 )
 def test_search_path(files: Path, nix: Nix, snapshot: Callable[[str], Snapshot]):