Make XCTMain test observations configurable

Currently, XCTest uses `PrintObserver` as a default observation. While it is possible to add other custom observations through `XCTestObservationCenter.shared.addTestObserver`, it doesn't prevent the default `PrintObserver` from printing the default output. In certain situations this is undesirable, especially when you'd like to get a custom observation output through stdout. This change allows passing a custom observation through a third new argument to `XCTMain` to replace the default observation. The existing two-argument overload is still available, so this doesn't have any impact on existing users.

Author: MaxDesiatov

Sources/XCTest/Public/XCTestMain.swift

@@ -62,7 +62,16 @@
 public func XCTMain(_ testCases: [XCTestCaseEntry]) -> Never {
     XCTMain(testCases, arguments: CommandLine.arguments)
 }
+
 public func XCTMain(_ testCases: [XCTestCaseEntry], arguments: [String]) -> Never {
+  XCTMain(testCases, arguments: arguments, observation: PrintObserver())
+}
+
+public func XCTMain(
+  _ testCases: [XCTestCaseEntry],
+  arguments: [String],
+  observation: XCTestObservation
+) -> Never {
     let testBundle = Bundle.main
 
     let executionMode = ArgumentParser(arguments: arguments).executionMode
@@ -128,7 +137,7 @@ public func XCTMain(_ testCases: [XCTestCaseEntry], arguments: [String]) -> Neve
     case .run(selectedTestNames: _):
         // Add a test observer that prints test progress to stdout.
         let observationCenter = XCTestObservationCenter.shared
-        observationCenter.addTestObserver(PrintObserver())
+        observationCenter.addTestObserver(observation)
 
         observationCenter.testBundleWillStart(testBundle)
         rootTestSuite.run()