xterm.tests.musl: init

This would have caught the build regression introduced by
3bf2356 ("xterm: 390 -> 392 (#311472)"), and fixed by
5b2bbd8 ("Revert "xterm: patch from alpine to fix pty on musl"").

Taking over from Alyssa Ross due to age of PR and merge conflicts.

Link: https://github.com/NixOS/nixpkgs/pull/327357

Co-developed-by: Alyssa Ross <hi@alyssa.is>
Signed-off-by: Ethan Carter Edwards <ethan@ethancedwards.com>

pkgs/by-name/xt/xterm/package.nix

@@ -9,6 +9,7 @@
   pkg-config,
   makeWrapper,
   nixosTests,
+  pkgsCross,
   gitUpdater,
   enableDecLocator ? true,
 }:
@@ -95,6 +96,7 @@ stdenv.mkDerivation rec {
     tests = {
       customTest = nixosTests.xterm;
       standardTest = nixosTests.terminal-emulators.xterm;
+      musl = pkgsCross.musl64.xterm;
     };
 
     updateScript = gitUpdater {