simpletap: simplify to use parent run method

Per the TODO, use `__init__` to override the stream. There don't seem to
be any other substantial changes to the `run` method needed for TAP and
I'm assuming the additional code in the upstream `run` has been added
since this class was written.

My primary motivation was to set `result.buffer = self.buffer` in the
`run` method, which fixes the runner enough to be able to use
`breakpoint()`.

In addition, I updated the shebang in `tw-2189.t` for portability.

test/tw-2189.t

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 . bash_tap_tw.sh
 
 task add "foo \' bar"