Incorrect code snippets in `core::testing` docs

[ddoktorski]

https://docs.starknet.io/build/corelib/core-testing-get_available_gas
https://docs.starknet.io/build/corelib/core-testing-get_unspent_gas

The assert! at the bottom is incorrect. It was probably supposed to be something like:

assert!(gas_after - gas_before < 100_000);