[CMake] CTest with multiple directories: problem with absolute paths

[Bill V]

> Question 3: Why *does* it work if I switch to using relative paths
> instead of absolute paths?
>
> If I edit my CTestTestfile.cmake to instead use a relative path to that
> same directory:
>
> SUBDIRS(../../../../path/to/unittest/_build/linux/debug)


The subdir code in CMAKE proper (cmSubdirCommand.cxx) has logic to
handle relative or absolute paths.   The subdir handling code in
CTest/cmCTestTestHandler.cxx  always assumes it's a relative path and
prepends the current directory.     Maybe cmCTestTestHandler.cxx needs
to be updated.

Bill