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 https://lab.nexedi.com/nexedi/erp5/merge_requests/1441.

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.