box: increase connection timeout in "net.box.test.lua"

The "box/net.box.test.lua" test contains a check that the error
received contains a 'timed out'. But in cases when testing was
conducted on a slow computer or in the case of a very large load,
it is possible that the connection time-out will be reached
earlier than the mentioned error. In this case, the error "Invalid
argument" will be returned. To prevent this from happening,
this patch will increase the connection timeout.

Closes #4341