gh-110167: Increase support.LOOPBACK_TIMEOUT to 10 seconds (#110413)

Increase support.LOOPBACK_TIMEOUT from 5 to 10 seconds. Also increase
the timeout depending on the --timeout option. For example, for a
test timeout of 40 minutes (ARM Raspbian 3.x), use LOOPBACK_TIMEOUT
of 20 seconds instead of 5 seconds before.