Fix a deprecation warning in DeclarationMacroTests

Author: ahoppen

Tests/SwiftSyntaxMacroExpansionTest/DeclarationMacroTests.swift

@@ -91,9 +91,9 @@ final class DeclarationMacroTests: XCTestCase {
         }
         """,
       diagnostics: [
-        DiagnosticSpec(message: "please don't do that", line: 1, column: 1, highlight: #"#myError("please don't do that")"#),
-        DiagnosticSpec(message: "#error macro requires a string literal", line: 4, column: 3, highlight: #"#myError(bad)"#),
-        DiagnosticSpec(message: "worse", line: 6, column: 5, highlight: #"#myError("worse")"#),
+        DiagnosticSpec(message: "please don't do that", line: 1, column: 1, highlights: [#"#myError("please don't do that")"#]),
+        DiagnosticSpec(message: "#error macro requires a string literal", line: 4, column: 3, highlights: [#"#myError(bad)"#]),
+        DiagnosticSpec(message: "worse", line: 6, column: 5, highlights: [#"#myError("worse")"#]),
       ],
       macros: ["myError": ErrorMacro.self],
       indentationWidth: indentationWidth