Move back to 100% coverage standard

In https://github.com/googleapis/google-cloud-python/issues/17456, it was reported that the coverage tests have been silently failing. We [opened a PR](https://github.com/googleapis/google-cloud-python/pull/17454) to address this, but left the required coverage at 99% instead of 100, since there will be a gap before the next generation cycle. After the generator is updated, we should move back to a 100% coverage standard.

There may be some hand-written package that will also fail the 100% check. We should make the CI check use package-specific .coverage files for this reason, so individual packages can set their own limits (with the goal of everyone reaching 100% eventually)