Move the stdout line-buffering call. (#576)

Move the `stdout` line-buffering call so it affects both overloads of
`__swiftPMEntryPoint()`.

This has no effect on SwiftPM because it currently uses the overload
that returns `Never`. Two overloads are provided to allow SwiftPM some
flexibility, but we're not actively taking advantage of that in Swift 6.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/ABI/EntryPoints/SwiftPMEntryPoint.swift

@@ -50,7 +50,16 @@ var EXIT_NO_TESTS_FOUND: CInt {
 /// - Warning: This function is used by Swift Package Manager. Do not call it
 ///   directly.
 public func __swiftPMEntryPoint(passing args: __CommandLineArguments_v0? = nil) async -> CInt {
-  await entryPoint(passing: args, eventHandler: nil)
+#if !SWT_NO_FILE_IO
+  // Ensure that stdout is line- rather than block-buffered. Swift Package
+  // Manager reroutes standard I/O through pipes, so we tend to end up with
+  // block-buffered streams.
+  FileHandle.stdout.withUnsafeCFILEHandle { stdout in
+    _ = setvbuf(stdout, nil, _IOLBF, Int(BUFSIZ))
+  }
+#endif
+
+  return await entryPoint(passing: args, eventHandler: nil)
 }
 
 /// The entry point to the testing library used by Swift Package Manager.
@@ -69,15 +78,6 @@ public func __swiftPMEntryPoint(passing args: __CommandLineArguments_v0? = nil)
 /// - Warning: This function is used by Swift Package Manager. Do not call it
 ///   directly.
 public func __swiftPMEntryPoint(passing args: __CommandLineArguments_v0? = nil) async -> Never {
-#if !SWT_NO_FILE_IO
-  // Ensure that stdout is line- rather than block-buffered. Swift Package
-  // Manager reroutes standard I/O through pipes, so we tend to end up with
-  // block-buffered streams.
-  FileHandle.stdout.withUnsafeCFILEHandle { stdout in
-    _ = setvbuf(stdout, nil, _IOLBF, Int(BUFSIZ))
-  }
-#endif
-
   let exitCode: CInt = await __swiftPMEntryPoint(passing: args)
   exit(exitCode)
 }