improve test time

[minrk]

• thin build matrix (avoid running all backends in any given run)
• show durations so we can see where the time is spent
• add some concurrency for slow calls like sequential add_route
• allow longer-running traefik/etcd/consul to avoid startup costs applied to every single test (note: only consul so far)

closes #148

[minrk]

This is already a huge improvement (~12 minutes overall instead of 60, not counting consul) and shows that something's up in consul, which is way slower than the rest.

[minrk]

Consul was by far the slowest, so I decided to make consul persistent, which reduced the test runtime for consul from 26 minutes to 8.

Making traefik persistent will be a bit of work because there will be several instances on different ports, so I'd rather do that in a separate PR. I think this is ready to go as an incremental improvement.

[GeorgianaElena]

Amazing @minrk ❤️ Thank you!