Require characters between dots in test paths

Our regular expression for test paths is a bit too permissive, and allows paths like these: `kselftest..arm64_syscall-abi_sched_yield_SME_VL_256_SM_ZA`. Tighten the regex in the next major schema release to require other characters between dots.

Start by locating the regular expression that validates test paths and the schema-release handling around it. Check how dot-separated path components are validated, then verify that the example containing consecutive dots is rejected while valid paths remain accepted in the next major schema release.

Written by the indexing model from the issue text.