Delete read only overlays before erasing buffer for an Org test

Some tests can cause the execution of asynchronous code blocks and
those overlays may not be removed before the start of the next Org
based test.

* jupyter-org-client.el (jupyter-org--make-overlay): Add `jupyter'
property to overlay.

* test/test-helper.el (jupyter-org-test-setup): Delete those overlays
before erasing the buffer.

jupyter-org-client.el

@@ -176,6 +176,7 @@ e.g. `org-babel-get-src-block-info'."
 (defun jupyter-org--make-overlay  (beg end &optional inline)
   "Create overlay between BEG and END positions and return it."
   (let ((overlay (make-overlay beg end)))
+    (overlay-put overlay 'jupyter t)
     (overlay-put overlay 'face 'secondary-selection)
     (let ((read-only
 	       (list

test/test-helper.el

@@ -501,6 +501,12 @@ should have PROP with VAL."
     (with-current-buffer jupyter-org-test-buffer
       (org-mode)))
   (with-current-buffer jupyter-org-test-buffer
+    ;; NOTE: Remove overlays introduced by
+    ;; `jupyter-org--make-overlay', these should have no bearing on if
+    ;; a test passes or not.
+    (dolist (ov (overlays-in (point-min) (point-max)))
+      (when (overlay-get ov 'jupyter)
+        (delete-overlay ov)))
     (erase-buffer)))
 
 (defun jupyter-org-test-client-from-info (info)