This fixes a test race condition when the start of the collector goroutine is delayed (e.g., on a loaded system).

In the original code, ticks are sent immediately upon return from Start. Unfortunately, there is a race condition due to timing: the collector's goroutine might not have reached the select statement when the ticks are sent.
The test timing is unpredictable:

• if the collector goroutine starts fast enough to catch both ticks before Stop is called, the test passes.
• otherwise, if The collector goroutine starts too slowly, it misses one or both ticks and the test fails.

The fix is in both the test and collector code:

• collector code is synchronized to return only after the goroutine has started (it waits on channel created inside Do())
• test uses require.Eventually (with a much shorter timeout compared to before)

/cc @ahg-g