first-push question: wait for input with select on macOS

On macOS /dev/tty takes no read deadline, so a prompt that timed out left a read blocked on the terminal, which took the next line typed, such as a git credential prompt during the upload. Wait with select before reading there. The test now reads through /dev/tty in a child whose controlling terminal is a pty; the pty's own device path takes deadlines and hid the bug.