Update event_handler_test to fake a gcs server

[sailorlqh]

Currently we use fake http response to fake response: https://github.com/abcxyz/pmap/blob/main/pkg/server/event_handler_test.go#L329

We can fake a server instead, like:

pmap/pkg/mapping/processors/assetinventory_test.go

Line 79 in 41c46ce

type fakeAssetInventoryServer struct {