Adding clean target for SDK conformance tests

[aLekSer]

Need to have a predefined make target to clean output
after running SDK conformance tests.

For #996 .

[agones-bot]

Build Failed 😱

Build Id: be42e051-0990-4f26-9ff9-4e6ca8bb2eda

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[aLekSer]

New E2E test fail:

time="2019-08-14 09:27:05.499" level=info msg="Scaling fleet" fleet=scale-fleet-0-gq8tg scale=0
time="2019-08-14 09:27:06.099" level=info msg="fleet simple-fleet-btv77 has 2/0 ready replicas"
time="2019-08-14 09:27:07.318" level=info msg="fleet simple-fleet-km7c6 has 2/0 ready replicas"
--- FAIL: TestGameServerShutdown (81.60s)
gameserver_test.go:368: 
Error Trace:	gameserver_test.go:368
Error: Received unexpected error:
timed out waiting for the condition
Test: TestGameServerShutdown

[markmandel]

Hit retry. TestGameServerShutdown is definitely showing up as the preeminent flaky test atm.

[agones-bot]

Build Succeeded 👏

Build Id: 6796329c-e6b7-417a-a1f8-4f6fbf432d8b

The following development artifacts have been built, and will exist for the next 30 days:

• image: gcr.io/agones-images/agones-controller:1.0.0-91420f2
• image: gcr.io/agones-images/agones-sdk:1.0.0-91420f2
• image: gcr.io/agones-images/agones-ping:1.0.0-91420f2
• Linux C++ SDK (build): agonessdk-1.0.0-91420f2-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.0.0-91420f2.zip

A preview of the website (the last 30 builds are retained):

• https://91420f2-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/GoogleCloudPlatform/agones.git pull/997/head:pr_997 && git checkout pr_997
• helm install install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.0.0-91420f2

[markmandel]

Looks good to me