test/test-lib.sh: fix two out of tree test issues

json_check_nodes.py exists in source tree, not in out of tree
build tree. Added -B to the execution so source tree is not
"polluted" by a .pyc file when json_check_nodes.py is executed.

When creating run_emacs.sh make it load .elc files from out of
tree build tree, not from source tree if such files existed.
If existed, those may be outdated, or even created by some other
emacs than the one that was used to build .elc files in out of
tree build dir.