doc/coding-conventions: Increase passthru test timeout 3s -> 60s

3s is too small a margin for a loaded slow system to start a bloated
program.
This leads to problems when tests are written on decent dev hardware
but later run on build farms of potentially slower hardware,
particularly in the case of non x86.

The chance of needing the timeout is actually very small, so those
rare 57s are a reasonable price to pay for fewer timeouts on build
farms, each of which should be investigated and usually fixed by
increasing the timeout.

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