Remove the time requirement for the statschannel truncated test [v9.18]

The 5 seconds requirement to finish the 'pipelined with truncated stream' was causing spurious failures in the CI because the job runners might be very busy and sending 128k of data might simply take some time.

Remove the time requirement altogether, there's actually no reason why the test SHOULD or even MUST finish under 5 seconds.

Backport of !6951 (merged)

Closes #3270 (closed)