wishbone.bus.Decoder: Only assert stb when slave is selected

[antonblanchard]

While a slave should check both cyc and stb are asserted, it is a bit
odd that we assert stb on all slaves even if they are not selected.

This changes stb to behave like the cyc signal, and adds a few tests.

[antonblanchard]

The wishbone spec mentions this about the stb signal:

The strobe input [STB_I], when asserted, indicates that the SLAVE is
selected. A SLAVE shall respond to other WISHBONE signals only when this
[STB_I] is asserted (except for the [RST_I] signal which should always
be responded to). The SLAVE asserts either the [ACK_O], [ERR_O] or
[RTY_O] signals in response to every assertion of the [STB_I] signal

[jfng]

Sorry for this very late response.

The Wishbone spec also says that interface signals are only valid if cyc is asserted:

3.1.2 Transfer Cycle initiation
MASTER interfaces initiate a transfer cycle by asserting [CYC_O]. When [CYC_O] is
negated, all other MASTER signals are invalid.
SLAVE interfaces respond to other SLAVE signals only when [CYC_I] is asserted.
SYSCON signals and responses to SYSCON signals are not affected.

AFAIU, this allows stb to be broadcast to all decoder subordinates to save some resources.

If this behavior is incompatible with other implementations, I don't mind changing it. Could you rebase this PR to main ?