FdtBusDxe: add test to time DT node traversal.

[andreiw]

Consider both a very deep hierarchy and repeated access to a particular node's properties. This is coming from a concern voiced from a 3rd party about overhead of parsing a giant DT tree, and may prompt optimizations to the FDT parsing code itself (which ultimately means taking in the libfdt code directly in instead of having this be a dependent on a Tiano library).