SmallVector: Improve test coverage for insert with repetition

To test cases that involve actual repetition (> 1 elements), at least
one element before the insertion point, and some elements of the
original range that still fit in that range space after insertion.

Actually we need coverage for the inverse case too (where no elements
after the insertion point fit into the previously allocated space), but
this'll do for now, and I might end up rewriting bits of SmallVector to
avoid that special case anyway.