Make _XCTIsTesting check a bit more robust (#21)

* Improve _XCTIsTesting

* wip

Author: stephencelis

Sources/XCTestDynamicOverlay/XCTIsTesting.swift

@@ -1,9 +1,9 @@
 import Foundation
 
 public let _XCTIsTesting: Bool = {
-  guard let path = ProcessInfo.processInfo.arguments.first
-  else { return false }
-
-  let url = URL(fileURLWithPath: path)
-  return url.lastPathComponent == "xctest" || url.pathExtension == "xctest"
+  ProcessInfo.processInfo.environment.keys.contains("XCTestSessionIdentifier")
+    || ProcessInfo.processInfo.arguments.first
+      .flatMap(URL.init(fileURLWithPath:))
+      .map { $0.lastPathComponent == "xctest" || $0.pathExtension == "xctest" }
+      ?? false
 }()