tests.nixpkgs-check-by-name: Test against sbcl-like regression

This adds a test to check that a commit like 0a3dab4af34e4d086931d82827bfc8760c3e3150 would fail CI

After doing some improvements to the `pkgs/by-name` check I discovered
that sbcl shouldn't have been allowed in `pkgs/by-name` after all as is.

Specifically, the requirement is that if `pkgs/by-name/sb/sbcl` exists,
the definition of the `sbcl` attribute must look like

    sbcl = callPackage ../by-name/sb/sbcl/package.nix { ... };

However it currently is an alias like

    sbcl = sbcl_2_4_1;

This wasn't detected before because `sbcl_2_4_1` was semantically
defined using `callPackage`:

    sbcl_2_4_1 = wrapLisp {
      pkg = callPackage ../development/compilers/sbcl { version = "2.4.1"; };
      faslExt = "fasl";
      flags = [ "--dynamic-space-size" "3000" ];
    };

However this doesn't syntactically match what is required.

In https://github.com/NixOS/nixpkgs/pull/285089 I introduced syntactic
checks for exactly this, but they were only used for packages not
already in `pkgs/by-name`.

Only now that I'm doing the refactoring to also use this check for
`pkgs/by-name` packages this problem is noticed.

While introducing this new check is technically an increase in
strictness, and therefore would justify adding a new ratchet, I consider
this case to be rare enough that we don't need to do that.

This commit introduces a test to prevent such regressions in the
future

Moving sbcl back out of `pkgs/by-name` will be done when the pinned CI is updated

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