IpSecManagerTest: clean up assertion for better error reporting

See the following flaky failure:
https://android-build.corp.google.com/test_investigate/?invocationId=I59900010160325881&testResultId=TR98428662107002537

Bug: 270193575
Test: TH
Change-Id: I90dfbbec49d86fbf61d378f0ae98ddde5f27bf1f
1 file changed