Fix mocking multiple http calls in the same function call (#6510)

* Fix mocking multiple http calls in the same function call

Fixes an issue where a function call would perform more than one http request and wait for each to complete before proceeding. The `RequestId` comes from the length of the `requests` collection in the `OffchainState` and if a request is completed before the next one starts it will be removed and the "next expected" will be off by one. This PR tries to fix that by using a request counter that tracks how many requests have been performed so that we can `remove()` items from the `expected_requests` at the right index.

I suspect that this is a sub-optimal soluton and perhaps requests and their mocks should live side by side in the same collection, e.g. in a tuple of `(PendingRequest, Option<ExpectedRequest>)`.

* Update primitives/core/src/offchain/testing.rs

Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>

* Update primitives/core/src/offchain/testing.rs

Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>

* Panic on overflow

* Update primitives/core/src/offchain/testing.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Use a Deque and push/pop expected requests

* fix test

Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/client/executor/src/integration_tests/mod.rs

@@ -497,9 +497,7 @@ fn offchain_http_should_work(wasm_method: WasmExecutionMethod) {
 	let mut ext = TestExternalities::default();
 	let (offchain, state) = testing::TestOffchainExt::new();
 	ext.register_extension(OffchainExt::new(offchain));
-	state.write().expect_request(
-		0,
-		testing::PendingRequest {
+	state.write().expect_request(testing::PendingRequest {
 			method: "POST".into(),
 			uri: "http://localhost:12345".into(),
 			body: vec![1, 2, 3, 4],