runtest: extract the logic that update timeout for slow tests
We want to use the "slow" test logic not only for "#require" clauses but also in
"#if" clauses. For this to be useful we need the logic to bump the timeout in at
least two spots. The first step it to factor it out.
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed