test(ldtestdata): increase channel-wait timeouts to reduce CI flakiness

[aaron-zeisler]

Summary

Increases hardcoded time.Millisecond channel-wait timeouts in testhelpers/ldtestdata to time.Second to fix intermittent CI flakiness.

Background

TestFlagTargets/user_targets intermittently failed in CI (observed on the Windows, Go 1.26 job) with:

channels.go:126: expected a mocks.UpsertParams value from channel but did not receive one in 1ms
channels.go:128: timed out before receiving expected update

Sibling jobs in the same CI run (Windows Go 1.24/1.25, all Linux jobs) passed with the identical test suite, confirming this is a timing flake rather than a real regression. Several test call sites in test_data_source_flag_test.go and test_data_source_test.go pass a hardcoded time.Millisecond timeout to WaitForUpsert/WaitForInit/WaitForNextInit/AssertChannelClosed, which wait on a channel populated by a producer goroutine. Under go test -race on slower/jitterier runners, 1ms isn't enough margin. This pattern dates back to a 2020 commit and has just been latent until now.

No behavior change is intended — only the timeout values increase, so tests will still fail fast (within a second) on a genuine hang.

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Note

Low Risk
Test-only timeout tuning with no production or behavioral changes; genuine hangs still fail within one second.

Overview
Raises channel-wait deadlines from time.Millisecond to time.Second in testhelpers/ldtestdata tests that block on mock data-source updates.

Affected helpers include WaitForUpsert, WaitForInit, WaitForNextInit, and AssertChannelClosed in test_data_source_test.go and the shared verifyFlag path in test_data_source_flag_test.go. Assertions and production code are unchanged; only how long tests wait before failing on a missing channel message.

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