IpManagerTest: fix flaky test for real

Commit c10fe5f4 tried to fix the flaky unit test for a default
ProvisioningConfiguration but failed to reinitialize the mock class
involved, so that in practice the flaky assertion is as flaky as before.

This patch correctly resets the mock after the default assertions are
done once an IpManager object is created.

Bug: 32561414
Bug: 62918393
Test: runtest frameworks-net
Change-Id: I516b6981f96e83a90cb372a727c41f0b2ea59801