The wlcopy_helper_succeeds_when_binary_returns_zero test used
`true` as a stand-in for wl-copy, but `true` exits immediately
without reading stdin. On Linux CI runners the process exits
before the parent can write to the pipe, producing EPIPE and
failing the is_ok() assertion.
Switch to `cat` which reads stdin until EOF (when the pipe is
closed) and then exits 0, faithfully modelling a successful
wl-copy invocation.
Hunter Bown
aa83446d6b