sync: fix pool wrap-around test
TestPoolDequeue in long mode does a little more than 1<<21 pushes.
This was originally because the head and tail indexes were 21 bits and
the intent was to test wrap-around behavior. However, in the final
version they were both 32 bits, so the test no longer tested
wrap-around.
It would take too long to reach 32-bit wrap around in a test, so
instead we initialize the poolDequeue with indexes that are already
nearly at their limit. This keeps the knowledge of the maximum index
in one place, and lets us test wrap-around even in short mode.
Change-Id: Ib9b8d85b6d9b5be235849c2b32e81c809e806579
Reviewed-on: https://go-review.googlesource.com/c/go/+/183979
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>