Add --experimental-attachments-path to swift test. (#8170)

This PR adds the given experimental command-line argument to `swift
test` to allow writing attachments to a user-supplied directory when
using Swift Testing. In the future, we will formally propose this flag
as part of the attachments feature.

XCTest on Darwin would need separate changes to support this argument
and they are beyond the scope of this PR. XCTest on non-Darwin platforms
(corelibs-xctest) does not support attachments.

Resolves rdar://141225464.

Author: grynspan

Sources/Commands/SwiftTestCommand.swift

@@ -119,6 +119,11 @@ struct TestEventStreamOptions: ParsableArguments {
     @Option(name: .customLong("event-stream-version"),
             help: .hidden)
     var eventStreamVersion: Int?
+
+  /// Experimental path for writing attachments (Swift Testing only.)
+  @Option(name: .customLong("experimental-attachments-path"),
+          help: .private)
+  var experimentalAttachmentsPath: AbsolutePath?
 }
 
 struct TestCommandOptions: ParsableArguments {