unflake TestAgent_ServiceHTTPChecksNotification

#### Overview of the Issue

`TestAgent_ServiceHTTPChecksNotification` will fail without apparent resource constraints. It popped it for me while working on another branch, and running a small sample (< 10) on current master was enough to get repeatable failures. On my laptop, the test completes in under a second and often under half a second.

`DONE 1000 tests, 56 failures in 524.106s`
Success rate is (very roughly) 94.4%

A common error condition (there may be more, but this is the one I've spotted) is:
`service_checks_test.go:68: got cache update for TTL check, expected timeout`

https://github.com/hashicorp/consul/blob/5934f803bfb54c1ceeeb6518398f1b82a726459f/agent/service_checks_test.go#L18

#### Reproduction Steps

`gotestsum --format short-verbose -- ./agent -run TestAgent_ServiceHTTPChecksNotification -timeout 0 -count 1000`

### Operating system and Environment details

Mac OS X 10.14.6

### Log Fragments

The entire test output and a sample of a single error are uploaded here:
https://gist.github.com/mkcp/86713958b4a0cf72923563d6c2e009fb