Fix testDhcpClientCaptivePortalApiEnabled flakes

The test expected onLinkPropertiesChanged to be called several times (it
iterates on all values received by the captor), but missed specifying
"atLeastOnce" in the verify calls, so only a single call would be
allowed.

This fixes frequent flakes in automated test runs.

Fixes: 163102019
Test: atest NetworkStackIntegrationTests:IpClientIntegrationTest
Change-Id: I27136451fc144c1df3fe42a3d51a0df2e9ff02ca