Add executable in the directoy containing the main program unit. This
is clearer as the executable file will be created in the corresponding
binary directory. Since the directory Overlap is added after
`Inst_eddies`, targets defined in `Overlap/Tests` are not yet defined
when processing `Inst_eddies`, so we cannot use `target_sources` for
these targets in `Inst_eddies`. So we use a relative path to
`Inst_eddies` from `Overlap/Tests`.