Re-write unit tests as integration tests

[jpflueger]

In a review for the fallback path feature, I received feedback to switch from std::env::var calls to spin_sdk::config::get calls for the configurable fallback path. Once I made this change, I was no longer able to run unit tests locally because the spin sdk only has the wasm32-wasi target. In order to use Spin Configuration, we should re-write the unit tests as integration tests because they currently only interact with the HTTP endpoint anyways and that will allow us to use spin's configuration. There will be another task for switching from environment variables to spin config but this issue is the predecessor to that work.

Source: #27