Test conic forms for constant values

[ericphanson]

Addresses https://github.com/jump-dev/Convex.jl/issues/616 in the real case only. In the complex case unfortunately I found many issues, too many to address in this PR. Some would be addressed similarly to the change for new_conic_form!(context::Context{T}, e::RelativeEntropyAtom), where GenericConstraint nicely handles the constant case, but direct MOI_add_constraint does not.

(The issues here are not really correctness bugs, but rather errors one would get if for some reason they had constant values rather than variables in some atoms, such as if you fix!'d a variable).

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.79%. Comparing base (ac382bb) to head (2088bb1).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #617 +/- ## ========================================== + Coverage 97.75% 97.79% +0.04% ========================================== Files 88 88 Lines 5073 5075 +2 ========================================== + Hits 4959 4963 +4 + Misses 114 112 -2 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[odow]

Okay. I've simplified the test handling, but now it requires #623

[odow]

Urgh. I borked that. Let's just wait and I'll rebase on master.