node-editor: Redo saving location

node-editor: Redo saving location

Determine the location to save testcases in dynamically,
trying first a GTK_SOURCE_DIR environment variable
and then the current directory as the GTK source dir,
ultimatively falling back to just saving in the current
directory.

This avoids leaking details of the build environment
into the produced artifacts and should make GTK builds
more reproducible.

Fixes: #5403