Missing regression test for calling `modify_symbolic_api` multiple times

[bradley-solliday-skydio]

At the moment, there is no protection against calling symforce.initialization.modify_symbolic_api multiple times.

However, this function is not idempotent, and will produce broken functions if applied to a sympy module multiple times. For example,

In [1]: import symforce; from symforce import sympy as sm

In [2]: symforce.set_backend("sympy"); symforce.set_backend("symengine")

In [3]: with sm.scope("dog"):
   ...:     print(sm.Symbol("a"))
   ...: 
dog.dog.a

produces a symbol named dog.dog.a instead of dog.a.

The fix for this is already in the pipeline, but it is not yet unit tested because the backend can not be changed from within a unit test. Instead, what likely needs to be done is to run the test in a subprocess.

We want this unit tested to ensure this issue doesn't pop up again (perhaps after a refactor or something).