Extend `testvalue` with a `::Type` argument

If this is a universal requirement, can you also document somewhere, perhaps on the definition of `testvalue` that it should produce a point from the sample space of the measure such that evaluating `logdensity` on it produces a `Float64`?

_Originally posted by @sethaxen in https://github.com/cscherrer/MeasureTheory.jl/pull/137#discussion_r688925536_

We should add methods to `testvalue` so that for any measure `d`,

1. `logdensity(d, testvalue(d)) isa Float64`
2. `logdensity(d, testvalue(T, d) isa T`