chore: Add a bazel test that calls toxic with --help.

This way we at least know it doesn't crash on load. Some code runs. We'll
want some real tests at some point, but this ensures *something* works.

BUILD.bazel

@@ -39,3 +39,9 @@ cc_binary(
         "//tools/config:osx": [],
     }),
 )
+
+sh_test(
+    name = "toxic_test",
+    srcs = [":toxic"],
+    args = ["--help"],
+)