Add a skip condition to all tests slower than 100ms.
This change was made using `gotestsum tool slowest` with data from the
last 3 CI runs of master.
See https://github.com/gotestyourself/gotestsum#finding-and-skipping-slow-tests
With this change:
```
$ time go test -count=1 -short ./agent
ok github.com/hashicorp/consul/agent 0.743s
real 0m4.791s
$ time go test -count=1 -short ./agent/consul
ok github.com/hashicorp/consul/agent/consul 4.229s
real 0m8.769s
```
This is in its own separate package so that it will be a separate test binary that runs thus isolating the go runtime from other tests and allowing accurate go routine leak checking.
This test would ideally use goleak.VerifyTestMain but that will fail 100% of the time due to some architectural things (blocking queries and net/rpc uncancellability).
This test is not comprehensive. We should enable/exercise more features and more cluster configurations. However its a start.
