Add TestingBuildParams.RelativeToTop()

The methods returns a copy of TestingBuildParams with every usage of a
path that is relative to the temporary test build directory with a path
relative to a notional top. Similar to how PathRelativeToTop does.

Bug: 182885307
Test: m nothing
Change-Id: I6ec20fc52ed76748138f0d48b4df80f765dfcfdc