Annotate the test observer with @MainActor where needed to be concu…

…rrency-safe.

PiperOrigin-RevId: 631886302

tools/test_observer/BazelXMLTestObserver.swift

@@ -29,6 +29,7 @@ public final class BazelXMLTestObserver: NSObject {
   ///
   /// If the `XML_OUTPUT_FILE` environment variable is not set or the file at that path could not be
   /// created and opened for writing, the value of this property will be nil.
+  @MainActor
   public static let `default`: BazelXMLTestObserver? = {
     guard
       let outputPath = ProcessInfo.processInfo.environment["XML_OUTPUT_FILE"],

tools/test_observer/BazelXMLTestObserverRegistration.swift

@@ -19,6 +19,7 @@
   /// The principal class in an XCTest bundle on Darwin-based platforms, which registers the
   /// XML-generating observer with the XCTest observation center when the bundle is loaded.
   @objc(BazelXMLTestObserverRegistration)
+  @MainActor
   public final class BazelXMLTestObserverRegistration: NSObject {
     @objc public override init() {
       super.init()