Add rand! method for AbstractRNG and AbstractArray types

Several functions invoke

    rand!(r::AbstractRNG, A::AbstractArray)

causing an error, since that method doesn't exist (issue #7643).
This commit provides the missing method and closes #7643.
Add test code for rand! with specified RNG

Adds a few tests that verify rand! executes when it's given
a specific RNG.
Merge pull request #7644 from grayclhn/randpatch

Add rand! method for AbstractRNG and AbstractArray types