This confused me fore a bit, because I thought the test was checking for
an actual `context.Canceled` error (which is spelled "context canceled"
with a single "l". But then I found that this was a string that's printed
as part of a test-utility, just looking very similar but with the British
spelling ("cancelled").
Let's change this to a message that's unique for the test, also to make it
more grep'able.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
c6b40640cc