test: Add more valgrind "possibly lost" suppressions

These valgrind entries are false positives because vim uses interior pointers.
channel: Delay notifications to avoid client race conditions

It is currently possible for a client to send a response that doesn't match the
current server->client request(at the top of the stack). This commit fixes that
by delaying notifications to until the first `channel_send_call` invocation
returns.

Also remove the "call stack" size check, vim will already break if the call
stack goes too deep.
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