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
```
Standardize naming
Use stricter assertions and reduce boilerplate to make the intent of the tests more obvious.
Also explicitly sort the filenames so that the correct files are pruned,
and so that the tests can not flake.
The tests all run fast enough that we do not get any advantage from
using Parallel.
The one test that was slow used a long sleep. Changing the sleep to
a few milliseconds speeds up the test considerably.
