Plotting deprecations in tests

[felixcremer]

Currently our tests throw the following deprecations below. We should try to switch to the new syntax.

│  WARNING: Makie.Combined is deprecated, use Plot instead.
│    likely near none:1
│  WARNING: importing deprecated binding Makie.Combined into CairoMakie.
│  , use Plot instead.
│  WARNING: Makie.ContinuousSurface is deprecated, use VertexGrid instead.
│    likely near none:1
│  WARNING: importing deprecated binding Makie.ContinuousSurface into CairoMakie.
│  , use VertexGrid instead.
│  WARNING: Makie.DiscreteSurface is deprecated, use CellGrid instead.
│    likely near none:1
│  WARNING: importing deprecated binding Makie.DiscreteSurface into CairoMakie.
│  , use CellGrid instead.
│  
│  [pid 5771] waiting for IO to finish:
│   Handle type        uv_handle_t->data
│   timer              0x25c4450->0x7f5f8b701990
│  This means that a package has started a background task or event source that has not finished running. For precompilation to complete successfully, the event source needs to be closed explicitly. See the developer documentation on fixing precompilation hangs for more help.
│  
│  [pid 5771] waiting for IO to finish:
│   Handle type        uv_handle_t->data
│   timer              0x25c4450->0x7f5f8b701990
│  This means that a package has started a background task or event source that has not finished running. For precompilation to complete successfully, the event source needs to be closed explicitly. See the developer documentation on fixing precompilation hangs for more help.
└  
┌ DimensionalData → DimensionalDataMakie
│  WARNING: Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).
│  You may need to write `f(x::Vararg{T})` rather than `f(x::Vararg{<:T})` or `f(x::Vararg{T}) where T` instead of `f(x::Vararg{T} where T)`.
│  WARNING: Wrapping `Vararg` directly in UnionAll is deprecated (wrap the tuple instead).
│  You may need to write `f(x::Vararg{T})` rather than `f(x::Vararg{<:T})` or `f(x::Vararg{T}) where T` instead of `f(x::Vararg{T} where T)`.
└  
┌ Plots
│  WARNING: using deprecated binding Colors.RGB1 in PlotUtils.
│  , use XRGB instead.
│  WARNING: using deprecated binding PlotUtils.RGB1 in Plots.
│  , use ColorTypes.XRGB{T} where T<:Union{AbstractFloat, FixedPointNumbers.FixedPoint{T, f} where f where T<:Integer} instead.
│  WARNING: using deprecated binding Colors.RGB4 in PlotUtils.
│  , use RGBX instead.
│  WARNING: using deprecated binding PlotUtils.RGB4 in Plots.
│  , use ColorTypes.RGBX{T} where T<:Union{AbstractFloat, FixedPointNumbers.FixedPoint{T, f} where f where T<:Integer} instead.

[asinghvi17]

Fixed by #795