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[mock] fix flaky TestOrderer by making SubmitEnv synchronous

Root cause: The inEnvs channel is buffered, so SubmitEnv returns
immediately without the Run goroutine having processed the envelope.
When CutBlock is called next, Go's select may pick the cutBlock case
before the inEnvs case, causing sendBlockData to operate on empty
data — no block is created and the test hangs.

Fix: Make SubmitEnv block until the envelope is collected by Run().
Introduce envelopeEntry which wraps an envelope with an optional
Ready signal. SubmitEnv (test helper) sends with a signal and waits;
Broadcast (gRPC) sends without one, preserving buffered throughput
for load generation.

CutBlock remains a simple fire-and-forget signal — it doesn't need
to wait since SubmitEnv already guarantees collection.

Signed-off-by: Senthilnathan <cendhu@gmail.com>