chore(NODE-3715): add code coverage generation to Evergreen tasks

[baileympearson]

Description

This PR adds code coverage generation for each build in Evergreen.

What is changing?

This PR does the following

• Adds a function to generate code coverage for each variant run in evergreen and upload it to s3 after each variant runs
• Adds a function (currently not used) to download all coverage reports for a patch, generate a combined coverage report and upload that to s3
• Adds a new build variant that depends on all other variants (currently commented out) to combine all coverage reports into one and upload it to s3 (calls the above function)

Is there new documentation needed for these changes?

Nope

Double check the following

• Ran npm run check:lint script
• Self-review completed using the steps outlined here
• PR title follows the correct format: <type>(NODE-xxxx)<!>: <description>
• Changes are covered by tests
• New TODOs have a related JIRA ticket

[nbbeeken]

This corresponds to nyc npm run test:all so the unit tests will be repeated on every variant. Maybe we want a new script for this to just run the int tests with coverage?

[baileympearson]

Yeah, good catch! Would we want a separate command to run our unit tests with coverage in CI as well? (I'm assuming we run the unit tests in CI at some already...?)

[nbbeeken]

Yes we run them 🙂 they're part of the lint task, so if you go to that script you make it run nyc in front of the unit run there, and it would also get the upload after task just fine I think?

[dariakp]

LGTM :)