Add Test.isRunning to let code determine if Swift Testing is active.

This PR adds a `Test.isRunning` static property that tells the caller if _any_ thread/task/queue/etc. is currently running Swift Testing code. This property is distinct from `Test.current` because that property has a value of `nil` on detached tasks.

Resolves #475.

Sources/Testing/Running/Runner.RuntimeState.swift

@@ -106,6 +106,14 @@ extension Configuration {
     _all.rawValue.instances.values
   }
 
+  /// The number of instances of this type that are currently set as the current
+  /// configuration for a task.
+  static var currentCount: Int {
+    _all.withLock { all in
+      all.instances.count
+    }
+  }
+
   /// Add this instance to ``Configuration/all``.
   ///
   /// - Returns: A unique number identifying `self` that can be
@@ -147,6 +155,9 @@ extension Test {
   /// by using [`Thread.detachNewThread(_:)`](https://developer.apple.com/documentation/foundation/thread/2088563-detachnewthread)
   /// or [`DispatchQueue.async(execute:)`](https://developer.apple.com/documentation/dispatch/dispatchqueue/2016103-async)),
   /// the value of this property may be `nil`.
+  ///
+  /// To determine if any test is running on any task in the current process,
+  /// use ``Test/isRunning``.
   public static var current: Self? {
     Runner.RuntimeState.current?.test
   }
@@ -165,6 +176,15 @@ extension Test {
     runtimeState.test = test
     return try await Runner.RuntimeState.$current.withValue(runtimeState, operation: body)
   }
+
+  /// Whether or not any test is running in the current process.
+  ///
+  /// To get an instance of ``Test`` representing the test running on the
+  /// current task, use ``Test/current``.
+  @_spi(Experimental)
+  public static var isRunning: Bool {
+    Configuration.currentCount > 0
+  }
 }
 
 extension Test.Case {

Tests/TestingTests/RunnerTests.swift

@@ -878,5 +878,14 @@ final class RunnerTests: XCTestCase {
     await runTest(for: DeprecatedVersionTests.self, configuration: configuration)
     await fulfillment(of: [testStarted, testSkipped], timeout: 0.0)
   }
+
+  func testTestIsRunning() async {
+    // Only XCTest is running here.
+    XCTAssertFalse(Test.isRunning)
+
+    await Test {
+      XCTAssertTrue(Test.isRunning)
+    }.run(configuration: .init())
+  }
 }
 #endif