tests: use a nested attrset for buildbot

We need to optimise the buildbot attrs for nix-eval-jobs, however this
doesn't make sense for `nix flake check` or `nix flake show`.

Now that we aren't using the `checks` output, we don't _need_ to
restrict ourselves to a flat set of test derivations anymore.

This also simplifies our `tests` command, and means we no longer need to
pre-compute the test attr names.

flake/dev/devshell.nix

@@ -59,26 +59,10 @@
 
                     text = builtins.readFile ./launch-test.sh;
                   };
-
-                  tests =
-                    let
-                      checks' = self'.checks;
-                      names = builtins.filter (n: builtins.match "test-.*" n != null) (builtins.attrNames checks');
-                    in
-                    builtins.listToAttrs (
-                      builtins.concatMap (
-                        checkName:
-                        map (testName: {
-                          name = testName;
-                          value = "${checkName}.entries.${testName}";
-                        }) (builtins.attrNames checks'.${checkName}.entries)
-                      ) names
-                    );
                 in
                 ''
                   export NIXVIM_SYSTEM=${system}
                   export NIXVIM_NIX_COMMAND=${nix}
-                  export NIXVIM_TESTS=${pkgs.writers.writeJSON "tests.json" tests}
                   ${lib.getExe launchTest} "$@"
                 '';
             }