Refactoring in commit v3.9.0-rc1~156^2 (c++: prefer vectors over lists,
2017-05-04) switched `cmCTestSVN::Repositories` from `std::list` to
`std::vector`.  This can cause re-allocation when svn externals are
processed and break the `RootInfo` pointer that is supposed to point at
the first repository element.  Switch back to `std::list` so that the
address remains stable.

Fixes: #17854