Jenkinsfile,Makefile: add coverage flag

[ihaid]

PR Checklist

• [x] The PR title is formatted as follows: pkg/server: add TestXYZ for XYZ
  • [x] The package name goes before the colon
  • [x] The part after the colon uses the verb tense + phrase that completes the blank in, "This change modifies Gateway-MT to ___________"
  • [x] Lowercase verb after the colon
  • [x] No trailing period
  • [x] Keep the title as short as possible. Ideally under 72 characters or shorter
  • [x] No Markdown

Commit Message Body

• What, why, please describe the tests and the performance impact (if any)
• No Markdown
• The body is wrapped at 72 characters unless it's really needed (ASCII art, table, or long link)
• If there is a corresponding issue, add either Closes #1234, Fixes #1234, Resolves #1234 or Updates #1234 (the latter if this is not a complete fix) to this comment
• If referring to a repo other than storj/gateway-mt, you can use the owner/repo#issue_number syntax: Fixes storj/common#1234
• We do not use Signed-off-by lines. Please don't add them. Our Gerrit server & GitHub bots enforce CLA compliance instead
• More: https://github.com/storj/docs/blob/main/code/Git.md
• Delete these instructions once you have read and applied them

Related Issue

Code Review Checklist (to be filled out by reviewer)

• [ ] Are there any authservice's database migrations? Are they forwards and backwards compatible?
• [ ] Does the PR describe what changes are being made?
• [ ] Does the PR describe why the changes are being made?
• [ ] Does the code follow our style guide?
• [ ] Does the code follow our testing guide?
• [ ] Is the PR appropriately sized? (If it could be broken into smaller PRs, it should be)
• [ ] Does the new code have enough tests? (every PR should have tests or justification otherwise. Bug-fix PRs especially)
• [ ] Does the new code have enough documentation that answers "how do I use it?" and "what does it do?"? (both source documentation and higher level, diagrams?)
• [ ] Does any documentation needs updating?
• [ ] Do the database access patterns make sense?
• [ ] Copy the Commit Message Body section contents into the submit prompt upon PR completion

[ihaid]

Closes #275