Creating a challenging backend to show the transpiler stochasticity (backport #12116)

[mergify]

The documentation about the transpiler shows that the process is stochastic in https://docs.quantum.ibm.com/api/qiskit/transpiler#routing-stage:

In order to highlight this, we run a GHZ circuit 100 times, using a “bad” (disconnected) initial_layout:


This distribution is quite wide, signaling the difficulty the swap mapper is having in computing the best mapping.

Which is not very wide. That's because GenericFakeBackend replaced a FakeBackend in #11376. The GenericFakeBackend has an all-to-all connectivity by default.

This PR produces the following image:

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This is an automatic backport of pull request #12116 done by [Mergify](https://mergify.com).

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