xtest: add test name sanitizer
xtest: add test name sanitizer

Add a `xtest.SanitizedName` helper function.

GitOrigin-RevId: 439fed81d3efb015e0a53db8c976ee19bcce23a6

netsec-ethz/scion,netsec-ethz/scion,netsec-ethz/scion,netsec-ethz/scion