Fixes #2103

Currently, TestHelper sets up / tears down tests by mutating the objects on the same layer. With this design, fixing #2103 would require storing the previous values of config and assigning them back after tests, which is error-prone.

So in this PR, I decided to utilise the SDK's layer design in setup_sentry_test:

1. We make sure the base layer's configuration uses dummy values.
2. And then we push another layer to the stack, which has a copied dummy configuration as well as a blank scope.
3. When teardown_sentry_test is called, we can simply pop the layer from 2.

If anything unexpected happens (e.g. users popped the stack themselves), having the base layer created in 1) will make sure no NoMethodError will be thrown or having any data being sent by accident.