tests: Add mock for virRandomBytes

tests: Add mock for virRandomBytes

Create a mock for virRandomBytes to generate a not so random value.
This should be usable by other tests that need a not so random number
to be generated by including the virrandommock at preload.

The "random number" generated is based upon the size of the expected
stream of bytes being returned where each byte in the result gets
the index of the array - hence a 4 byte array returns 0x00010203.