unflake TestAgent_AddCheck_MissingService in CI

#### Overview of the Issue

https://github.com/hashicorp/consul/blob/ba8fd8296fe7c69615d0d27afdb2bbf71b6b32df/agent/agent_test.go#L1239

Did not fail locally in 1k runs, try with resource contention `make test-flake`

#### Reproduction Steps

Did not fail in 1k runs on local
`gotestsum --format short-verbose --jsonfile TestAgent_AddCheck_MissingService.json -- ./agent -run TestAgent_AddCheck_MissingService -count 1000`

### Operating system and Environment details

CircleCI, Mac OS X 10.14.6

### Log Fragments

CI: https://circleci.com/gh/hashicorp/consul/58322

Start with TestAgent_AddCheck_MissingService in agent/agent_test.go around line 1239. Run the provided gotestsum command and try make test-flake to investigate behavior under resource contention, then compare with the linked CircleCI failure. Done means the test no longer flakes in CI or under the reported contention conditions.

Written by the indexing model from the issue text.