Isolate AttributeRemoverTests to AttributeRemover

* Add a private assert helper: `assertSyntaxRemovingTestAttributes`
* Make `AttributeRemover` SPI (`@_spi(Testing)`)
* Rather than depending on `SyntaxNode`'s id-only `Equatable`
  conformance, refactor `AttributeRemover` to remove attributes matching
  a predicate: `AttributeRemover.init(removingWhere:)` of type
  `(AttributeSyntax) -> Bool)`. This change allows `AttributeRemover` to
  be used without first plucking the `AttributeSyntax` values to be
  removed from a live tree.

Note: Unlike `assertMacroExpansion`, the new
`assertSyntaxRemovingTestAttributes` does not trim newlines before
comparing values. The trimming by `assertMacroExpansion` was masking a
(minor) bug in `AttributeRemover` where an extra leading newline remains
when the removed attribute has no proceeding tokens. For now, this
commit "fixes" the failing tests by including the unwanted leading
newlines in the expected test output, while also adding "FIXME" comments
to draw attention to the need for a future fix.

Author: gohanlon

Sources/SwiftSyntaxMacroExpansion/MacroSystem.swift

@@ -384,19 +384,24 @@ struct MacroSystem {
 // MARK: - MacroApplication
 
 /// Removes attributes from a syntax tree while maintaining their surrounding trivia.
-private class AttributeRemover: SyntaxRewriter {
-  var attributesToRemove: [AttributeSyntax]
+@_spi(Testing)
+public class AttributeRemover: SyntaxRewriter {
+  let predicate: (AttributeSyntax) -> Bool
 
   var triviaToAttachToNextToken: Trivia = Trivia()
 
-  init(attributesToRemove: [AttributeSyntax]) {
-    self.attributesToRemove = attributesToRemove
+  /// Initializes an attribute remover with a given predicate to determine which attributes to remove.
+  ///
+  /// - Parameter predicate: A closure that determines whether a given `AttributeSyntax` should be removed.
+  ///   If this closure returns `true` for an attribute, that attribute will be removed.
+  public init(removingWhere predicate: @escaping (AttributeSyntax) -> Bool) {
+    self.predicate = predicate
   }
 
-  override func visit(_ node: AttributeListSyntax) -> AttributeListSyntax {
+  public override func visit(_ node: AttributeListSyntax) -> AttributeListSyntax {
     var filteredAttributes: [AttributeListSyntax.Element] = []
     for case .attribute(let attribute) in node {
-      if attributesToRemove.contains(attribute) {
+      if self.predicate(attribute) {
         var leadingTrivia = attribute.leadingTrivia
 
         // Don't leave behind an empty line when the attribute being removed is on its own line,
@@ -450,7 +455,7 @@ private class AttributeRemover: SyntaxRewriter {
     return AttributeListSyntax(filteredAttributes)
   }
 
-  override func visit(_ token: TokenSyntax) -> TokenSyntax {
+  public override func visit(_ token: TokenSyntax) -> TokenSyntax {
     return prependAndClearAccumulatedTrivia(to: token)
   }
 
@@ -573,7 +578,7 @@ private class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
 
       let attributesToRemove = self.macroAttributes(attachedTo: visitedNode).map(\.attributeNode)
 
-      return AttributeRemover(attributesToRemove: attributesToRemove).rewrite(visitedNode)
+      return AttributeRemover(removingWhere: { attributesToRemove.contains($0) }).rewrite(visitedNode)
     }
 
     return nil