tests.nixpkgs-check-by-name: Test non-Nix invalid symlinks instead

Nix symlinks that are invalid don't pass the new CI check from
https://github.com/NixOS/nixpkgs/pull/279892, so let's instead use
non-Nix symlinks.

pkgs/test/nixpkgs-check-by-name/tests/multiple-failures/expected

@@ -1,6 +1,6 @@
 pkgs/by-name/A: Invalid directory name "A", must be at most 2 ASCII characters consisting of a-z, 0-9, "-" or "_".
 pkgs/by-name/A/fo@: Invalid package directory name "fo@", must be ASCII characters consisting of a-z, A-Z, 0-9, "-" or "_".
-pkgs/by-name/A/fo@: Path foo.nix is a symlink which cannot be resolved: No such file or directory (os error 2).
+pkgs/by-name/A/fo@: Path foo is a symlink which cannot be resolved: No such file or directory (os error 2).
 pkgs/by-name/A/fo@: Path package.nix is a symlink pointing to a path outside the directory of that package.
 pkgs/by-name/aa: This is a file, but it should be a directory.
 pkgs/by-name/ba/bar: This path is a file, but it should be a directory.

pkgs/test/nixpkgs-check-by-name/tests/multiple-failures/pkgs/by-name/A/fo@/foo

@@ -0,0 +1 @@
+none

pkgs/test/nixpkgs-check-by-name/tests/multiple-failures/pkgs/by-name/A/fo@/foo.nix

@@ -1 +0,0 @@
-../../../../foo.nix

pkgs/test/nixpkgs-check-by-name/tests/symlink-invalid/expected

@@ -1 +1 @@
-pkgs/by-name/fo/foo: Path foo.nix is a symlink which cannot be resolved: No such file or directory (os error 2).
+pkgs/by-name/fo/foo: Path foo is a symlink which cannot be resolved: No such file or directory (os error 2).

pkgs/test/nixpkgs-check-by-name/tests/symlink-invalid/pkgs/by-name/fo/foo/foo

@@ -0,0 +1 @@
+none

pkgs/test/nixpkgs-check-by-name/tests/symlink-invalid/pkgs/by-name/fo/foo/foo.nix

@@ -1 +0,0 @@
-none.nix