This will let a non-shared Theia (for example coming from shacache
binary) to be able to use the shared parts of a machine.

See merge request nexedi/slapos!1409

See merge request nexedi/slapos!1407

See merge request !1399

Fix takoever of a theia with initial embedded instance. The issue was
that the promise checking that the embedded instance was successfully
requested relied on files which were not transferred to backup theia.

The location of these files has been changed in order to transfer them;
therefore the old location is still checked for upgrade-compatibility.

Test that takeover of theia with an initial embedded instance succeeds.