[IMP] web: patch/unpatch: allow to re-apply a patch

In tests, we might want to disable a specific patch for a specific
test (e.g. to remove a monkey patch done when another addon is
installed, to test the behavior of the current addon).

Before this commit, it was possible to unpatch at the beginning of
the test, but we couldn't re-patch when the test was over.

Feature required by task~2392303

doc/reference/javascript_reference.rst

@@ -2811,10 +2811,14 @@ Therefore, patching a component constructor is always possible:
 Removing a patch
 -----------------
 
-We can also remove the patch by calling the ``unpatch`` function. This is really
-only useful for tests.
+In tests, it is sometimes useful to remove a patch for a specific test, for instance,
+to remove a patch applied when another addon is installed, which overrides the behavior
+implemented in the addon where the test is defined. The function ``unpatch`` exists for
+that purpose. It returns the removed patch, such that it can be re-applied at the end
+of the test.
 
 .. code-block:: javascript
 
-  unpatch(object, "patch name");
-
+  const p = unpatch(object, "patch name");
+  // test stuff here
+  patch(object, "patch name", p);