certificates: move db test to separate file

[bryanchriswhite]

What:

• Move DB related tests to db_test.go instead of authorizations_test.go
• Rename MaxClockOffset to MaxClockSkew as per @egonelbre's suggestion

Why: Better organization.

Please describe the tests: N/A

Please describe the performance impact: N/A

Code Review Checklist (to be filled out by reviewer)

• NEW: Are there any Satellite 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 need updating?
• Do the database access patterns make sense?