Ensure TestHttpChannel.close() is idempotent

[thecoop]

Closeable.close states that close is idempotent - you can call it on an already-closed object and it doesn't do anything. This change makes TestHttpChannel.close() follow this contract.

[elasticsearchmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[thecoop]

I can see an argument for leaving it as-is - it's for testing purposes only, and close shouldn't be called multiple times, as that might indicate a bug. But it is valid to call close multiple times, so should TestHttpChannel allow it?

[DaveCTurner]

While I agree that throwing an ISE is the wrong reaction to double-closing, I'd rather we asserted that this never happens. Yes technically these things should be idempotent according to the JDK docs, but in practice we should consider it a logic error if we're double-closing stuff.

[thecoop]

I've added an assertion failure to the close.

In practice, asserts are always turned on for tests, so this will still fail, but it turns the failure into an AssertionError which is more fitting what we want here

[DaveCTurner]

LGTM

[thecoop]

@elasticmachine update branch