Add (String)Path(s)RelativeToTop and assert functions

The existing NormalizePathForTesting function does not handle make
install paths very well, as it returns a relative path with a leading
"../" which is very confusing. It also does not clearly differentiate
between the different paths.

These functions return paths that are basically what are seen in a
normal developer build, i.e.
* <source path>
* out/soong/<soong output path>
* out/<make output path>

That makes tests that use them easier to understand.

Follow up changes will clean up the existing usages of the
Normalize... functions.

Bug: 182885307
Test: m nothing
Change-Id: I17ddc996bef5bbbf4a62da8334ea6ce29e306109