tests/neotest: avoid deprecated vim.tbl_flatten

Use plain tables (and vim.iter flatten where needed).

tests/test-sources/plugins/by-name/neotest/elixir.nix

@@ -18,7 +18,7 @@
             args = [ "--trace" ];
             post_process_command.__raw = ''
               function(cmd)
-                return vim.tbl_flatten({{"env", "FOO=bar"}, cmd})
+                return vim.iter({ { "env", "FOO=bar" }, cmd }):flatten():totable()
               end
             '';
             write_delay = 1000;

tests/test-sources/plugins/by-name/neotest/minitest.nix

@@ -11,12 +11,12 @@
           settings = {
             test_cmd.__raw = ''
               function()
-                return vim.tbl_flatten({
+                return {
                   "bundle",
                   "exec",
                   "rails",
                   "test",
-                })
+                }
               end
             '';
           };

tests/test-sources/plugins/by-name/neotest/rspec.nix

@@ -11,11 +11,11 @@
           settings = {
             rspec_cmd.__raw = ''
               function()
-                return vim.tbl_flatten({
+                return {
                   "bundle",
                   "exec",
                   "rspec",
-                })
+                }
               end
             '';
             root_files = [