fix(test): stop Yahoo API test panic from aborting the whole suite

[NeoCN]

Problem

TestYahooCheckByAPI uses assert.NoError, which records a failure but does not halt. When
check() returns an error, execution continues to res.HostExists with res == nil and
panics, which kills the test binary.

That test is #44 of ~90 in the package, so every test after it stops running. Since the
upstream Yahoo endpoint broke around 2025-12-09 (see #195), the following have not executed in
CI for ~8.5 months:

• all of smtp_test.go — every SMTP and catch-all test
• all of suggestion_test.go
• all of util_test.go
• all of verifier_test.go

Every ci.yml run since 2025-12-09 has failed, and the panic meant the reported failure told
you about one test while silently hiding 47 others.

Change

One file, two commits. require.NoError instead of assert.NoError, pass the subtest's own
*testing.T, and switch the bool comparisons to assert.True/assert.False:

• require instead of assert — fails the subtest immediately rather than continuing on
  to dereference a nil *SMTP.
• tt instead of t — the closure parameter was declared but never used (legal for
  parameters, so it compiled), so assertions were attributed to the parent test. It also
  matters for require: FailNow must be called from the goroutine running the test, and
  subtests run in their own.
• assert.True/assert.False — testifylint's bool-compare rule rejects
  assert.Equal(tt, true, x). Those lines already violated it, but only-new-issues: true
  meant it was never surfaced until this branch touched them.

Verification

Run locally with the same flags CI uses, go test -race -covermode atomic ./...:

	before	after
outcome	panic at test #44	suite completes, 27.4s
tests never executed	47	0
failures	1 reported, 47 hidden	2, both explained

The two remaining failures share one root cause — yahoo check by api, no sessionIndex:

• TestYahooCheckByAPI
• TestCheckSMTPOK_ByApi (smtp_test.go:67, which goes through EnableAPIVerifier(YAHOO);
  it never panicked only because it does not dereference the nil result)

Spot-checked that the previously-dead tests genuinely pass rather than merely run: a targeted
run over verifier_test.go, smtp_test.go, suggestion_test.go and util_test.go gives 21
passes and ok.

Confirmed on CI as well — the run on this branch reaches the Test step, completes in 11.7s
with no panic, and reports those same two failures and nothing else. So the SMTP tests in
smtp_test.go do pass on GitHub's runners; they had simply not been reached since December.

Scope

This does not make CI green. The two tests above will keep failing until there is a
decision on the Yahoo verifier itself — #195 lays out the options (remove it following the
Gmail precedent in #113, chase Yahoo's replacement endpoint, or give catch_all a way to
express "undetermined"). That decision is deliberately out of scope here, since removing
EnableAPIVerifier(YAHOO) would be a breaking public API change.

What this does do is restore CI signal for the other 88 tests, so unrelated PRs stop carrying
a red check that hides whether they actually broke anything. #188 is a current example — the
test it adds lives in verifier_test.go and has never executed.

Refs #195

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