Backport of Fix panicky xDS test flakes into release/1.15.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #16305 to be assessed for backporting due to the inclusion of the label backport/1.15.

The below text is copied from the body of the original PR.

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Description

I'm seeing a bunch of panics in some flaky testing code in our xDS package. The relevant stack traces that come up time and again seem to mostly be on ARM (I'm assuming that this is due to some sort of slowness in the test runs that are accentuating race conditions due to additional qemu translation? 🤷):

goroutine 353 [running]:
github.com/hashicorp/consul/agent/xds.(*TestADSDeltaStream).Context(0x400009ff48?)
	<autogenerated>:1
github.com/hashicorp/consul/agent/xds.(*Server).DeltaAggregatedResources(0x4001581620, {0x2998370, 0x0})
	/home/circleci/project/agent/xds/delta.go:50 +0x4c
github.com/hashicorp/consul/agent/xds.newTestServerDeltaScenario.func3()
	/home/circleci/project/agent/xds/xds_protocol_helpers_test.go:197 +0x38
created by github.com/hashicorp/consul/agent/xds.newTestServerDeltaScenario
	/home/circleci/project/agent/xds/xds_protocol_helpers_test.go:196 +0x408

Notice the DeltaAggregatedResources(0x4001581620, {0x2998370, 0x0}) -- the first argument should be the pointer address to the xds server itself, the second is the first argument of the DeltaAggregatedResources function (stream ADSDeltaStream), which, since it's an interface is a tuple of the type information and then the actual pointer to the underlying type, which is nil.

We seem to be niling the argument that gets passed there in our test helpers while cleaning up after a test run, which, if any requests are in flight will pretty much cause any of our handler code to panic and the tests to abort (hence the ridiculous amount of flake).

Really we should probably fix the testing code so it doesn't nil something that's often unconditionally dereferenced and also potentially used in a multithreaded context (I wouldn't be surprised if there were still panics due to only nil checking once and using it later in the function), but this is a simple attempt to at least make the tests fail less often, and I'd prefer this for now than to potentially heavily refactor the testing code (I'm also not sure what tests rely on the nil behavior of the stream).

Edit: Introduced a boolean guard in the test code itself that should hopefully fix the panics since we were still hitting them due to this being multithreaded. Doesn't appear like anything relies on the nil, so hopefully that should be fine.

PR Checklist

• updated test coverage
• external facing docs updated
• not a security concern

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Overview of commits

• 01ca6e2 - 485cb4d

[github-team-consul-core-pr-approver]

Auto approved Consul Bot automated PR