[TaskCenter][5/n] Introduces restate_core::test macro

Introduces a new macro that creates a default TaskCenter in tests. This should replace usage of `tokio::test` (the code is mostly based on it) and it provides access to `TaskCenter::current()` within the test code.

This also introduces a transitional type `TestCoreEnv2`, a clone of `TestCoreEnv` but without TaskCenter. After migration, this will be renamed to `TestCoreEnv` again.

Cargo.toml

@@ -2,6 +2,7 @@
 members = [
     "cli",
     "crates/*",
+    "crates/core/derive",
     "crates/codederror/derive",
     "server",
     "benchmarks",
@@ -14,6 +15,7 @@ members = [
 default-members = [
     "cli",
     "crates/*",
+    "crates/core/derive",
     "crates/codederror/derive",
     "server",
     "tools/restatectl",
@@ -38,6 +40,7 @@ restate-base64-util = { path = "crates/base64-util" }
 restate-bifrost = { path = "crates/bifrost" }
 restate-cli-util = { path = "crates/cli-util" }
 restate-core = { path = "crates/core" }
+restate-core-derive = { path = "crates/core/derive" }
 restate-errors = { path = "crates/errors" }
 restate-fs-util = { path = "crates/fs-util" }
 restate-futures-util = { path = "crates/futures-util" }