test: simplify sleep-and-wait pairs with synctest.Sleep

[dgageot]

Replace seven time.Sleep(d); synctest.Wait() pairs with Go 1.27's synctest.Sleep(d), which is documented as exactly equivalent.

• Simplify five pairs in source-loader tests and one each in streaming and delegation tests.
• Remove seven unnecessary //nolint:forbidigo suppressions and their fake-time justifications.
• Preserve durations, leading waits, standalone sleeps, assertions, and timeout-boundary comments. No production-code changes.

Validation

• CI=true task build and task lint pass (0 lint issues).
• Affected tests pass 20 times with -race -shuffle=on.
• With .env.test loaded, go test -p 4 ./... passes when only the two DMR example-loading subtests are skipped.
• Unfiltered task test fails on TestLoadExamples for dmr.yaml and unload_on_switch.yaml because Docker Model Runner is not running locally. Both failures reproduce with the unchanged base files through a Go overlay.
• A highly parallel filtered rerun also hit process-start timeout failures in codingharness and evaluation; both passed in the lower-parallelism rerun.