Fix multiline comments not rendering correctly at CLI. (#850)

This PR fixes a bug where a multi-line comment or message written to
`stderr` would only have its first line in grey and would not write
atomically, resulting in odd/incorrect output. All messages generated by
a test event are now written to `stderr` atomically.

Resolves rdar://134519515.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/Events/Recorder/Event.ConsoleOutputRecorder.swift

@@ -304,18 +304,40 @@ extension Event.ConsoleOutputRecorder {
   ///   destination.
   @discardableResult public func record(_ event: borrowing Event, in context: borrowing Event.Context) -> Bool {
     let messages = _humanReadableOutputRecorder.record(event, in: context)
-    for message in messages {
-      let symbol = message.symbol?.stringValue(options: options) ?? " "
 
-      if case .details = message.symbol, options.useANSIEscapeCodes, options.ansiColorBitDepth > 1 {
+    // Padding to use in place of a symbol for messages that don't have one.
+    var padding = " "
+#if os(macOS) || (os(iOS) && targetEnvironment(macCatalyst))
+    if options.useSFSymbols {
+      padding = "  "
+    }
+#endif
+
+    let lines = messages.lazy.map { [test = context.test] message in
+      let symbol = message.symbol?.stringValue(options: options) ?? padding
+
+      if case .details = message.symbol {
         // Special-case the detail symbol to apply grey to the entire line of
-        // text instead of just the symbol.
-        write("\(_ansiEscapeCodePrefix)90m\(symbol) \(message.stringValue)\(_resetANSIEscapeCode)\n")
+        // text instead of just the symbol. Details may be multi-line messages,
+        // so split the message on newlines and indent all lines to align them
+        // to the indentation provided by the symbol.
+        var lines = message.stringValue.split(whereSeparator: \.isNewline)
+        lines = CollectionOfOne(lines[0]) + lines.dropFirst().map { line in
+          "\(padding) \(line)"
+        }
+        let stringValue = lines.joined(separator: "\n")
+        if options.useANSIEscapeCodes, options.ansiColorBitDepth > 1 {
+          return "\(_ansiEscapeCodePrefix)90m\(symbol) \(stringValue)\(_resetANSIEscapeCode)\n"
+        } else {
+          return "\(symbol) \(stringValue)\n"
+        }
       } else {
-        let colorDots = context.test.map(\.tags).map { self.colorDots(for: $0) } ?? ""
-        write("\(symbol) \(colorDots)\(message.stringValue)\n")
+        let colorDots = test.map { self.colorDots(for: $0.tags) } ?? ""
+        return "\(symbol) \(colorDots)\(message.stringValue)\n"
       }
     }
+
+    write(lines.joined())
     return !messages.isEmpty
   }
 

Sources/Testing/Events/Recorder/Event.HumanReadableOutputRecorder.swift

@@ -92,19 +92,7 @@ extension Event.HumanReadableOutputRecorder {
   /// - Returns: A formatted string representing `comments`, or `nil` if there
   ///   are none.
   private func _formattedComments(_ comments: [Comment]) -> [Message] {
-    // Insert an arrow character at the start of each comment, then indent any
-    // additional lines in the comment to align them with the arrow.
-    comments.lazy
-      .flatMap { comment in
-        let lines = comment.rawValue.split(whereSeparator: \.isNewline)
-        if let firstLine = lines.first {
-          let remainingLines = lines.dropFirst()
-          return CollectionOfOne(Message(symbol: .details, stringValue: String(firstLine))) + remainingLines.lazy
-            .map(String.init)
-            .map { Message(stringValue: $0) }
-        }
-        return []
-      }
+    comments.map { Message(symbol: .details, stringValue: $0.rawValue) }
   }
 
   /// Get a string representing the comments attached to a test, formatted for