node-editor: Save test data relative to current working directory
This avoids hard-coding the GTK source directory into the binary, which would make the binary non-reproducible across different source paths, particularly in a distro build environment where the source directory used by autobuilders will often not exist on end-user systems. The node editor can still be used to create new test-cases by running it with the GTK source directory as its current working directory.
Resolves: #5403 (closed)
See #5403 (closed) for some possible alternatives to this.