[Docs][TENT] Document the FakeTransport runtime test mechanism (#3489)

Explain why a fake Transport can drive the unmodified TENT runtime,
how fakes are swapped in and wrapped for fault injection, and what
this can and cannot prove. Keep failover.md pointed at that mechanism
instead of a per-case inventory.

Signed-off-by: staryxchen <staryxchen@tencent.com>