pw_containers: Test more duplicates for multimap

IntrusiveMultiMap's unit tests were developed from IntrusiveMap's. The
test fixture for the latter contained all unique items. In order to
batter test the "multi" part of the multimap, this CL modifies to
fixture to contain duplicates.

Change-Id: I413bae190f41196521127fd389806f0fc5848b18
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/243256
Docs-Not-Needed: Aaron Green <[email protected]>
Reviewed-by: Wyatt Hepler <[email protected]>
Lint: Lint 🤖 <[email protected]>
Commit-Queue: Aaron Green <[email protected]>