Fix offset when widening collections

[timholy]

The final argument of copyto! is a count; subtracting 1 from the current index works only if indexing starts at 1. This also explicitly uses LinearIndices just as occurs in upstream callers.

[timholy]

It should be noted that the destination array typically supports linear indexing, and so none of our current tests check this code for IndexCartesian destination types. Consequently, I decided to move the TSlow type from test/abstractarray.jl to test/testhelpers/ so that it can be reused. It also seemed to be an opportune moment to update it given the merging of #11242. A trip down memory lane with @mbauman 😄.