Clean up auction_worklet_manager_unittest.cc a little.

In particular:
* The last two tests had incorrect URL parameters. Fix them. The tests
were still valid, just had the wrong cross-origin URL as the script URL
vs the signals URL.
* Destroy AuctionProcessManager in TearDown(). AuctionWorkletManagerTest
inherits from RenderViewHostTestHarness, which tears down the
ScopedTaskEnvironment on shutdown. AuctionProcessManager can own
BidderWorklets and SellerWorklets, which spin the task runner on
destruction, so they cannot be destroyed after the task environment is
shut down. The new destruction order means tests don't have to take
ownership and destroy and worklets they don't care about.

Bug: None
Change-Id: Ifb14c26b00e8deafa03b69b2f34b5c24371f12cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6039697
Reviewed-by: Russ Hamilton <behamilton@google.com>
Commit-Queue: mmenke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1386276}