Update formatting test so we can check that only whitespace edits are produced

Change the test case to something a little more realistic than lines containing A, B, and C. Then add the check again that all changes reported are whitespaces, which was remvoed by #2291.

Author: ahoppen

Tests/SourceKitLSPTests/FormattingTests.swift

@@ -256,14 +256,14 @@ final class FormattingTests: XCTestCase {
       _ = [
         Node(
           documentation: """
-          A
-          B
-          C
+          Some great documentation
+          of an amazing
+          syntax node
           """,
           children: [
             Child(
               documentation: """
-              A
+              The one and only child
               \#("")
               """
             )
@@ -283,21 +283,22 @@ final class FormattingTests: XCTestCase {
 
     let edits = try XCTUnwrap(response)
     let formattedSource = apply(edits: edits, to: source)
+    XCTAssert(edits.allSatisfy { $0.newText.allSatisfy(\.isWhitespace) })
 
     XCTAssertEqual(
       formattedSource,
       #"""
       _ = [
         Node(
           documentation: """
-            A
-            B
-            C
+            Some great documentation
+            of an amazing
+            syntax node
             """,
           children: [
             Child(
               documentation: """
-                A
+                The one and only child
 
                 """
             )