Expose the flush bus interface of the CacheFiber

[inochisa]

Context, Motivation & Description

Expose the flushBus interface of the CacheFiber fiber so the caller can send FlushCmd via this (For example, The LsuPlugin of the VexiiRiscv).

Also add this parameter to all existed CacheFiber test.

Impact on code generation

Checklist

• Unit tests were added
• API changes are or will be documented:
  • using Scaladoc comments: /** */?
  • on RTD?
  • thanks to a new tracking issue on RTD?

[inochisa]

I guess the test time should be extended, The CacheTester cost more than 1.5 hour to finish in my machine.