test: stabilize concurrent JsVar broadcast assertion

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Summary

• Run the concurrent JsVar broadcast test in testing/synctest so both writes reach the same batch before the update tick.
• Replace the wall-clock wait with a channel handshake and a virtual tick; require exactly one final Set and the matching Go value.

The old assertion could read a valid intermediate Set when the writes straddled two ticks.

Verification

• Focused race test: 1,000 runs
• Full JAWS_REQUIRE_NODE=1 go test -race -count=1 ./... and non-race suite
• go vet, staticcheck, golangci-lint, gosec, gofumpt, go build

Fixes #408