Expose Test Reset Function

Our library can run both inside and outisde the InvokeStore async context. As such, I have suites of concurrency tests to verify theconcurrency behiours I expect to see. After the change in `v0.2.x` to change the instantiation logic, ie., the introduction of `getInstanceAsync` this has made testing much more difficult.

This is because the new `_testing` field is only added if the `AWS_LAMBDA_BENCHMARK_MODE` is set to 1 at the very first time the module is imported. This means that if `InvokeStore` is imported at the top level anywhere before your test, i.e., in application code, then the `_testing.reset` [function](https://github.com/awslabs/aws-lambda-invoke-store/blob/db30e3f22032026fa9e79bfd019d7842543e7ec9/src/invoke-store.ts#L186) will not be there. Using `vi.strubEnv` in a `before` block will not work because it is too late, the module has long been imported before then.

I propose that this `reset` function should always be available without the need for `AWS_LAMBDA_BENCHMARK_MODE` to allow for ease of testing for codebases that need to support both environments where `InvokeStore` needs to be use to manage async contexts and where it does not