Fix `testvalue` for `Counting` measures

As of v0.14.7, we have
```julia
testvalue(::Type{T}, d::Counting) where {T} = testvalue(T, d.support)
```

But then we get
```julia
julia> testvalue(Counting(Float64))
ERROR: MethodError: no method matching rand(::FixedRNG, ::Type{Float64}, ::Counting{Type{Float64}})

Closest candidates are:
rand(::FixedRNG, ::Type{Float64})
@ MeasureBase ~/git/MeasureBase.jl/src/fixedrng.jl:15
rand(::FixedRNG, ::Type{T}) where T<:Real
@ MeasureBase ~/git/MeasureBase.jl/src/fixedrng.jl:8
rand(::Random.AbstractRNG, ::Type{T}, ::StdLogistic) where T
@ MeasureBase ~/git/MeasureBase.jl/src/standard/stdlogistic.jl:13
...

Stacktrace:
[1] rand(rng::FixedRNG, d::Counting{Type{Float64}})
@ MeasureBase ~/git/MeasureBase.jl/src/rand.jl:7
[2] testvalue(μ::Counting{Type{Float64}})
@ MeasureBase ~/git/MeasureBase.jl/src/utils.jl:14
[3] top-level scope
@ REPL[8]:1
```

Currently this causes MeasureTheory tests to fail. So we should
1. Decide what the value of `rand(rng, T::Type, U::Type)` should be
2. Move tests like `testinterface(Counting(Float64))` from MeasureTheory to MeasureBase