Fix false-negative test case for __bp_install

This is an oversight of changing `trap DEBUG` to `trap - DEBUG` in

#106

The oversight did not cause a test error because the test always
succeeds.  The problem is that even if __bp_install is broken and
fails to remove `trap - DEBUG`, the current test case failed to detect
the failure and produce a false negative.  This patch fixes it.

test/bash-preexec.bats

@@ -79,7 +79,7 @@ set_exit_code_and_run_precmd() {
 
   eval_PROMPT_COMMAND
 
-  [[ "$PROMPT_COMMAND" != *"trap DEBUG"* ]] || return 1
+  [[ "$PROMPT_COMMAND" != *"trap - DEBUG"* ]] || return 1
   [[ "$PROMPT_COMMAND" != *"__bp_install"* ]] || return 1
 }