StdlibUnittest: add a workaround for Windows \r\n

Windows uses carriage returns and newlines.  The string matching is
sensitive to this.  Simply erase carriage returns from the standard
error stream to compensate.

Author: compnerd

stdlib/private/StdlibUnittest/StdlibUnittest.swift

@@ -928,7 +928,10 @@ class _ParentProcess {
     let (_, stderrThread) = _stdlib_thread_create_block({
       while !self._childStderr.isEOF {
         self._childStderr.read()
-        while let line = self._childStderr.getline() {
+        while var line = self._childStderr.getline() {
+          if let cr = line.firstIndex(of: "\r") {
+            line.remove(at: cr)
+          }
           var done: Bool
           (done: done, ()) = onStderrLine(line)
           if done { return }