test(formatters): FailNow on the running subtest, not the parent

[nikolauspschuetz]

Summary

Test_FmtOutput runs each formatter/feature combination as a t.Run(...) subtest, and the scenario/step hooks registered in fmtOutputTest call assert.FailNow(tT, ...) when they find unexpected attachments. But tT is a package-level *testing.T set once to the parent Test_FmtOutput:

var tT *testing.T

func Test_FmtOutput(t *testing.T) {
    tT = t
    ...
    t.Run(testName, fmtOutputTest(...))   // hooks inside call FailNow(tT)

So a hook running inside a subtest calls FailNow on the parent test, from the subtest's goroutine. testing documents that FailNow must be called from the goroutine running the test it belongs to; calling it on the parent from a child goroutine can misattribute the failure or mishandle the Goexit, rather than cleanly failing the specific subtest that broke.

Fix

Assign tT to the current subtest's *testing.T at the start of each returned test function, so FailNow targets the subtest that is actually running, on its own goroutine:

return func(t *testing.T) {
    tT = t
    ...

Subtests here run serially (no t.Parallel()) and godog runs the scenarios serially (no Options.Concurrency), so the shared tT is only ever written/read for the one subtest in flight — no data race.

Verification

• go test ./internal/formatters/ — passes (ok ... 0.764s); no behaviour change on the passing path.
• gofmt / go vet clean.

This is a correctness fix on the failure path (a mis-targeted FailNow only manifests when a hook assertion actually fails), so it does not change the green-path output; the value is that a real failure now fails the right subtest cleanly.