SeedNetworkPoliciesTest fails always

[rfranzke]

From gardener-attic/gardener-extensions#293

What happened:
The test defined in SeedNetworkPoliciesTest.yaml fails always.
Most of the time the following 3 specs fail:

2019-07-29 11:32:33	Test Suite Failed
2019-07-29 11:32:33	Ginkgo ran 1 suite in 3m20.280138435s
2x		2019-07-29 11:32:33	
2019-07-29 11:32:32	FAIL! -- 375 Passed | 3 Failed | 0 Pending | 126 Skipped
2019-07-29 11:32:32	Ran 378 of 504 Specs in 85.218 seconds
2019-07-29 11:32:32	
2019-07-29 11:32:32	> /go/src/github.com/gardener/gardener/test/integration/seeds/networkpolicies/aws/networkpolicy_aws_test.go:1194
2019-07-29 11:32:32	[Fail] Network Policy Testing egress for mirrored pods elasticsearch-logging [AfterEach] should block connection to "Garden Prometheus" prometheus-web.garden:80
2019-07-29 11:32:32	
2019-07-29 11:32:32	/go/src/github.com/gardener/gardener/test/integration/seeds/networkpolicies/aws/networkpolicy_aws_test.go:1062
2019-07-29 11:32:32	[Fail] Network Policy Testing components are selected by correct policies [AfterEach] gardener-resource-manager
2019-07-29 11:32:32	
2019-07-29 11:32:32	/go/src/github.com/gardener/gardener/test/integration/seeds/networkpolicies/aws/networkpolicy_aws_test.go:1194
2019-07-29 11:32:32	[Fail] Network Policy Testing egress for mirrored pods gardener-resource-manager [AfterEach] should block connection to "External host" 8.8.8.8:53

@mvladev can you please check?

Environment:
TestMachinery on all landscapes (dev, ..., live)