erp5testnode: Shorten instance partition paths
Rename some directories generated by erp5testnode to shorten the paths of the test partitions:
srv/testnode/abc/inst/test0-0 -> t/abc/i/0
This depends on modifications to the erp5.util testnode, see erp5!1441 (merged).
In total this saves 20 chars, which, when also using __partition_reference__, happens to be exactly enough to test a Theia containing an ERP5 without hitting the socket path length limit.
Also: replace softwaretype recipe with switch-softwaretype, and adapt instance templates accordingly.