Tests for Conversion space promotion

[codecov[bot]]

Codecov Report

Patch coverage: 19.04% and project coverage change: +0.03% :tada:

Comparison is base (27c3236) 27.41% compared to head (814ab60) 27.45%. Report is 3 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #540 +/- ## ========================================== + Coverage 27.41% 27.45% +0.03% ========================================== Files 80 80 Lines 8363 8370 +7 ========================================== + Hits 2293 2298 +5 - Misses 6070 6072 +2 ``` | [Files Changed](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/540?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/Multivariate/ProductFun.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/540?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL011bHRpdmFyaWF0ZS9Qcm9kdWN0RnVuLmps) | `0.00% <0.00%> (ø)` | | | [src/eigen.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/540?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL2VpZ2VuLmps) | `0.00% <0.00%> (ø)` | | | [src/Spaces/QuotientSpace.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/540?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL1NwYWNlcy9RdW90aWVudFNwYWNlLmps) | `3.95% <28.57%> (+0.50%)` | :arrow_up: | | [src/Operators/banded/Conversion.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/540?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL09wZXJhdG9ycy9iYW5kZWQvQ29udmVyc2lvbi5qbA==) | `26.98% <100.00%> (+4.03%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/540/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation)

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