REGTESTS: cache: validate the emission of 103s

REGTESTS: cache: validate the emission of 103s

Add four regtests to validate the behavior of the HTTP cache when early
hints are enabled.

- early_hints.vtc: covers 103 emission on stripped entries and that the
  "no-early-hints" rule keyword suppresses it.
- early_hints_ratio.vtc: covers cap-driven eviction of hints entries when
  the ratio is reached.
- early_hints_parsing.vtc: pins down the (deliberately strict) Link
  header parsing behaviour of link_is_hint() so future relaxations
  cannot silently change hint detection.
- early_hints_multiblock.vtc: checks that hints are still extracted when
  the relevant Link headers span more than one cache block.