Increase startup timeout for servers in system tests

This change is made in particular to address the issue with 'doth'
system tests where servers are unable to iniitalise in time in CI
system under high load (that happened particularly often for Debian
Buster cross32 configuration).

The right solution, is, of course, to (re)use TLS context sparingly,
while right now we create too many of them.
67 jobs for !5609 with 2983-fix-doth-system-test in 19 minutes and 53 seconds (queued for 16 seconds)
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