nixos/tests/terminal-emulators: fix test for st

some change in the last 24 hours altered the behaviour of st such that
it now dies with a non-zero exit code when the shell exits, so kill is
now necessary

nixos/tests/terminal-emulators.nix

@@ -77,6 +77,7 @@ let tests = {
       sakura.pkg = p: p.sakura;
 
       st.pkg = p: p.st;
+      st.kill = true;
 
       stupidterm.pkg = p: p.stupidterm;
       stupidterm.cmd = "stupidterm -- $command";