[Toybox] avoiding testing shell builtins

[enh]

i realized today that the kill tests are testing the shell's builtin
kill, not the toybox kill. one solution would be to use env, but
presumably that would be better in the infrastructure rather than
having to remember to do it in each test that might be subverted by a
builtin?

diff --git a/tests/kill.test b/tests/kill.test
index a91dd55a..78d0b742 100644
--- a/tests/kill.test
+++ b/tests/kill.test
@@ -4,5 +4,5 @@

 #testing "name" "command" "result" "infile" "stdin"

-testing "-l HUP" "kill -l HUP" "1\n" "" ""
-testing "-l 1" "kill -l 1" "HUP\n" "" ""
+testing "-l HUP" "env kill -l HUP" "1\n" "" ""
+testing "-l 1" "env kill -l 1" "HUP\n" "" ""