* test: route fossil-binary tests through one REQUIRE_FOSSIL_BIN helper

REQUIRE_FOSSIL_BIN=1 turns a missing fossil binary into a test failure instead of an invisible skip, but only internal/manifest honored it. The six other fossil-dependent sites each did their own exec.LookPath + unconditional t.Skip, so on those a run with no fossil was byte-identical to a passing one.

Add testutil.RequireFossilBin, which resolves the binary the same way FossilBinary does (FOSSIL_BIN override, then PATH) and either skips or fails based on REQUIRE_FOSSIL_BIN. Route testutil.NewTestRepo, both internal/delta CLI round-trips, the four internal/merge validation tests, the internal/xfer sanity check, and the ls-branch-tip repro through it, deleting their bespoke lookup+skip logic. CI already sets REQUIRE_FOSSIL_BIN=1 on the unit-test steps, so this makes all seven sites fail loud on absence while local runs without fossil stay opt-in.

* test: route every fossil-binary gate through RequireFossilBin

The first pass left six sites still deciding for themselves whether a missing fossil binary is a skip or a failure. Three of them (internal/sync's startFossilServer and its two integration tests, manifest's two rebuild tests, the root TestPhaseB_Integration, and hash's TestSHA1_FossilValidation) skipped unconditionally, so with REQUIRE_FOSSIL_BIN=1 and no fossil on PATH those packages still reported ok -- the exact silent pass the flag exists to prevent. The other three had hand-rolled copies of the flag check, which is the duplication that let the original drift happen.

All of them now call testutil.RequireFossilBin, and HasFossil is gone: it was a second, skip-only way to ask the same question, and it is what produced the sites that ignored the flag.

Also validate the FOSSIL_BIN override with LookPath, so a stale path reports "fossil not found" instead of surfacing later as a raw fork/exec error.

* test: resolve fossil binary via RequireFossilBin at remaining call sites

* fix: correct stale module path missed by rebase auto-merge

Three imports added by earlier commits in this branch predated the rebase target and had no corresponding line on main to conflict against, so git's auto-merge carried the old module path through verbatim. check-in: f804878bd3 user: [email protected] tags: trunk