Code Coverage: store lcov report

[bayandin]

Problem

In the future, we want to compare code coverage on a PR with coverage on the main branch.
Currently, we store only code coverage HTML reports, I suggest we start storing reports in "lcov info" format that we can use/parse in the future. Currently the file size is ~7Mb (it's a text-based format and could be compressed into a ~400Kb archive)

• More about "lcov info" format: https://manpages.ubuntu.com/manpages/jammy/man1/geninfo.1.html#files
• Part of Revive code coverage usage in neon repo #3543

Summary of changes

• Change scripts/coverage to output lcov coverage to report/lcov.info file instead of stdout (we already upload the whole report/ directory to S3)

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

1004 tests run: 964 passed, 0 failed, 40 skipped (full report)

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Flaky tests (2)

Postgres 15

• test_get_tenant_size_with_multiple_branches: ✅ release
• test_pageserver_lsn_wait_error_safekeeper_stop: ✅ release

_{The comment gets automatically updated with the latest test results
f26f9b0 at 2023-05-26T16:07:23.323Z :recycle:}