`assertMacroExpansion()` gives different diagnostics than using a macro when attached to multiple variables

### Description

This is reproduced with swift-syntax@603.0.0.

When testing macros attached to properties where there are multiple properties on the same `var/let`, `assertMacroExpansion()` gives an error before executing the macro. But when using the macro outside of `assertMacroExpansion()`, the macro is executed similarly to when there is only one property.

---

Testing macro

Using macro outside test

### Steps to Reproduce

A minimal project showing the issue is here: https://github.com/fizker/swift-syntax-peer-macro-issue

## To summarize:

We start with the following macro:

```swift
public struct MyMacro: PeerMacro {
public static func expansion(
of node: AttributeSyntax,
providingPeersOf declaration: some DeclSyntaxProtocol,
in context: some MacroExpansionContext,
) throws -> [DeclSyntax] {
return []
}
}
```

We then use the macro
```swift
struct Foo {
@MyMacro
var a: Int, b: Int
}
```

If this code is placed in regularly executing Swift, it runs without issues and the return-statement is hit twice, for both `a` and `b`. But if it executed through `assertMacroExpansion()`, a diagnostic is raised stating that `peer macro can only be applied to a single variable`, and the macro code is never executed.