Format tree itself instead of its description

Author: gibachan

Tests/SwiftBasicFormatTest/BasicFormatTests.swift

@@ -351,7 +351,7 @@ final class BasicFormatTest: XCTestCase {
     let body = decl.cast(FunctionDeclSyntax.self).body!
 
     assertFormatted(
-      source: body.formatted().description,
+      tree: body,
       expected: """
         {
             print(1)
@@ -372,7 +372,7 @@ final class BasicFormatTest: XCTestCase {
     let body = decl.cast(StructDeclSyntax.self).memberBlock.members.first!.decl.cast(FunctionDeclSyntax.self).body!
 
     assertFormatted(
-      source: body.formatted().description,
+      tree: body,
       expected: """
         {
                 print(1)