Fix an issue when running tests on Windows with Python 3.6 and later.

Since Python 3.6 on Windows, io.IOBase object overlaid tty is incompatible
with other file like objects. This makes it impossible to redirect input or
output between file and tty without changing file descriptor number by using
os.dup2() without changing the overlaid object [1].

This prevents our test suite from running correctly because it attempts to
redirect stdout to a file using os.dup2() [2].

This is a workaround for it.

[1] _io._WindowsConsoleIO breaks in the face of fd redirection
    https://bugs.python.org/issue30555

[2] https://lists.apache.org/thread.html/r87a33f0046949fe59188e844832297d8a00e675c4e59a1ab01005e0e%40%3Cdev.subversion.apache.org%3E

* build/run_tests.py
  (rebuild_textio): New.
  (TestHarness._run_py_test): Rebuild IO object after os.dup2() if it is
    needed.

Found by: jcorvel

Index: build/run_tests.py
===================================================================
--- build/run_tests.py	(revision 1876712)
+++ build/run_tests.py	(working copy)
@@ -48,6 +48,7 @@
 '''
 
 import os, sys, shutil, codecs
+import io
 import re
 import logging
 import optparse, subprocess, threading, traceback
@@ -141,6 +142,10 @@
   else:
     return s.decode("latin-1")
 
+def rebuild_textio(fd, mode):
+  '''Rebuild an io.BaseIO object from file descriptor for sys.std*'''
+  return open(fd, mode, encoding='utf-8', errors='replace', closefd=False)
+
 class TestHarness:
   '''Test harness for Subversion tests.
   '''
@@ -844,6 +849,16 @@
 
     # setup the output pipes
     if self.log:
+      # Since Python 3.6 on Windows, redirecting tty to file or file to tty
+      # by using os.dup2 breaks overlaid IO object.
+      # (See https://bugs.python.org/issue30555).
+      # To avoid this, we rebuild IO object after os.dup2 if it is needed.
+      need_rebuild_stdout = (    isinstance(sys.stdout, io.IOBase)
+                             and not isinstance(sys.stdout.buffer.raw,
+                                                io.FileIO))
+      need_rebuild_stderr = (    isinstance(sys.stderr, io.IOBase)
+                             and not isinstance(sys.stderr.buffer.raw,
+                                                io.FileIO))
       sys.stdout.flush()
       sys.stderr.flush()
       self.log.flush()
@@ -851,6 +866,10 @@
       old_stderr = os.dup(sys.stderr.fileno())
       os.dup2(self.log.fileno(), sys.stdout.fileno())
       os.dup2(self.log.fileno(), sys.stderr.fileno())
+      if need_rebuild_stdout:
+        sys.stdout = rebuild_textio(sys.stdout.fileno(), 'w')
+      if need_rebuild_stderr:
+        sys.stderr = rebuild_textio(sys.stderr.fileno(), 'w')
 
     # These have to be class-scoped for use in the progress_func()
     self.dots_written = 0
@@ -897,6 +916,10 @@
       os.dup2(old_stderr, sys.stderr.fileno())
       os.close(old_stdout)
       os.close(old_stderr)
+      if need_rebuild_stdout:
+        sys.stdout = rebuild_textio(sys.stdout.fileno(), 'w')
+      if need_rebuild_stderr:
+        sys.stderr = rebuild_textio(sys.stderr.fileno(), 'w')
 
     return failed