bug: Fix unreliable tests in coverage builds, and re-enable coverage reports

[teor2345]

Scheduling

We haven't had coverage for a year and none of our developers think it is an important priority, so this ticket is optional.

What happened?

I expected to see this happen:

Tests consistently pass in the coverage workflow.

Instead, this happened:

Some tests fail in unrelated PRs, including:
primitives::redjubjub::tests::batch_flushes_on_max_items_test
primitives::redjubjub::tests::batch_flushes_on_max_latency_test
primitives::redpallas::tests::batch_flushes_on_max_items
primitives::redpallas::tests::batch_flushes_on_max_latency
transaction::tests::v4_with_modified_joinsplit_is_rejected

What were you doing when the issue happened?

Making an unrelated change in PR #7819.

Zebra logs

failures:
primitives::redjubjub::tests::batch_flushes_on_max_items_test
primitives::redjubjub::tests::batch_flushes_on_max_latency_test
primitives::redpallas::tests::batch_flushes_on_max_items
primitives::redpallas::tests::batch_flushes_on_max_latency
transaction::tests::v4_with_modified_joinsplit_is_rejected

test result: FAILED. 111 passed; 5 failed; 0 ignored; 0 measured; 0 filtered out; finished in 72.27s

https://github.com/ZcashFoundation/zebra/actions/runs/6645922313/job/18058933174#step:7:2868

Zebra Version

zebrad 1.3.0 PR #7819

Which operating systems does the issue happen on?

• Linux
• macOS
• Windows
• Other OS

OS details

CI

Additional information

I have turned off the coverage workflow and the coverage branch protection rule.

After this PR merges, coverage needs to be restored:

• Re-enable https://github.com/ZcashFoundation/zebra/actions/workflows/ci-coverage.yml
• Admin: add a branch protection rule for the coverage job

[mpguerra]

We need to figure out what kind of code quality metrics we want to provide and ensure that they are accurate.

Un-scheduling again for now.

[mpguerra]

This doesn't seem to be a priority