`assertMacroExpansion` should emit an error if macro is applied to node that is not a variable

When applying an accessor macro to e.g. a struct, `assertMacroExpansion` will currently happily swallow the attribute. It should, however, emit an error that accessor macros can’t be applied to structs. Ie. the following test case should emit an error.

```swift

func testAccessorOnStruct() {
struct TestMacro: AccessorMacro {
static func expansion(
of node: AttributeSyntax,
providingAccessorsOf declaration: some DeclSyntaxProtocol,
in context: some MacroExpansionContext
) throws -> [AccessorDeclSyntax] {
return []
}
}

assertMacroExpansion(
"@Test struct Foo {}",
expandedSource: "struct Foo {}",
macros: ["Test": TestMacro.self]
)
}
```

rdar://115562563