Fix bug in convex yield collection

[DanielVF]

What this PR Does

1. We weren't actually collecting convex rewards. This fixes that.
2. The vault can currently only sell one token per strategy, and must have an oracle. Since there is not currently an oracle for CVX, this sets the strategy reward token to CRV which we can sell.
3. The address for CVX was set incorrectly to the LP token for the convex three pool. Fixed to be be CVX.
4. Tests were not testing that we collected rewards from upstream. Fixed.

Checklist

If you made a contract change, make sure to complete the checklist below before merging it in master.

Refer to our documentation for more details about contract security best practices.

Contract change checklist:

• Code reviewed by 2 reviewers. See template and example.
• Unit tests pass
• Slither tests pass with no warning
• Echidna tests pass if PR includes changes to OUSD contract (not automated, run manually on local)

[DanielVF]

Fork testing:

[franckc]

LGTM