Test accuracy of shadows technique

[Misty-W]

Issue Description

Shadows tests didn't catch inaccuracy bug discovered in docs: https://github.com/unitaryfund/mitiq/pull/2113#issuecomment-1863625596

Min also points to a possible culprit in another comment in that thread: https://github.com/unitaryfund/mitiq/pull/2113#issuecomment-1865243585

Proposed Solution

Additional References

[Misty-W]

Assigning myself to specify what is needed to solve this issue. I had just created this issue quickly to flag what was mentioned in https://github.com/unitaryfund/mitiq/pull/2113#issuecomment-1863625596, but I do recognize I need to follow up here.

*If someone wants to investigate and fill in the issue description before I get a chance to do it, more than welcome! Otherwise I'll pick it up.