fix: combine coverage and CI workflow

[shaneutt]

What this PR does / why we need it:

Our codecov integration is noisy and gets multiple profiles at different times, causing it to drop multiple messages on PRs.

Additionally, we want to be able to capture coverage output from multiple different test types (unit, integration, e.t.c.)

This PR fixes these problems by having each test target in the Makefile provide it's own coverage profile, and adds a CI workflow which will wait for all tests to finish before combining them and uploading them to codecov.

Additionally this enables the use of our codecov API token instead of doing anonymous updates.

[mflendrich]

The coverage CI run on this PR had a silent failure that we missed:

[2021-08-05T23:19:08.024Z] ['error'] There was an error reading the coverage file: Error: EISDIR: illegal operation on a directory, read
[2021-08-05T23:19:08.024Z] ['error'] There was an error reading the coverage file: Error: EISDIR: illegal operation on a directory, read
[2021-08-05T23:19:08.024Z] ['error'] There was an error reading the coverage file: Error: EISDIR: illegal operation on a directory, read

Reported #1699 following Travis's slack report and fixing this in #1700.