Filter events the env exposes in test snapshots to contracts to contr…

…act events (#1361)

### What
Filter events the env exposes in test snapshots to contracts to contract
events.

### Why
The main thing we need to test in test snapshots is that non-diagnostic
events are consistent and captured.

Diagnostic events are noise here.

### Notes
There's no good time to make this change.

Ideally the change is made at a major version because it's breaking.

But at a major version is the ideal time to avoid the change so that
people see only meaningful diffs on their test snapshots.

soroban-sdk/src/env.rs

@@ -1587,6 +1587,17 @@ impl Env {
                 .unwrap()
                 .0
                 .into_iter()
+                .filter(|e| match e.event.type_ {
+                    // Keep only system and contract events, because event
+                    // snapshots are used in test snapshots, and intended to be
+                    // stable over time because the goal is to record meaningful
+                    // observable behaviors. Diagnostic events are observable,
+                    // but events have no stability guarantees and are intended
+                    // to be used by developers when debugging, tracing, and
+                    // observing, not by systems that integrate.
+                    xdr::ContractEventType::System | xdr::ContractEventType::Contract => true,
+                    xdr::ContractEventType::Diagnostic => false,
+                })
                 .map(Into::into)
                 .collect(),
         )