test: replace polling loops and injectable-time overrides with synctest

Replace assert.Eventually polling loops, time-variable overrides, and
runtime.Gosched() workarounds with deterministic synctest bubbles.

- ddtrace/tracer/abandonedspans_test: wrap all subtests in
  synctest.Test, compute span start times relative to the fake clock's
  time.Now(), and remove the setTestTime() helper that overrode the
  package-level now var.
- internal/telemetry/client_test: wrap TestClientFlush in
  synctest.Test; add a 1 ns sleep in the heartbeat case to advance the
  fake clock past the rate-limiter interval; remove runtime.Gosched()
  calls (no-op inside bubbles); switch from t.Cleanup to defer for
  c.Close() so the ticker goroutine stops before the bubble exits.
- ddtrace/tracer/tracer_test: migrate TestTracerAtomicFlush and
  TestWorker; replace a time.After+time.Sleep polling loop with
  synctest.Wait() followed by a direct assertion.

Signed-off-by: Kemal Akkoyun <kemal.akkoyun@datadoghq.com>

Author: kakkoyun