The test previously asserted only wall-clock elapsed < 2s, which passes
whether or not the drain is skipped because the fake stream ends
immediately. Record any read past the terminal frame and assert there
were none, so re-enabling the drain without a shutdown-capable socket
fails deterministically. Also inline the single-use _post_terminal_socket
helper at its call site (keeping the exception guard, since the drain
sits outside the inner try; _socket_from_response already handles a
None response), and drop `x and` / bare-truthiness preambles that the
following set-equality asserts already imply.
cdb8fd3ef2