regrtest doesn't ignore -j1 anymore

* regrtest now uses subprocesses when the -j1 command line option
  is used: each test file runs in a fresh child process. Before, the -j1 option
  was ignored.
* Tools/buildbot/test.bat script now uses -j1 by default to run
  each test file in fresh child process.

Lib/test/libregrtest/cmdline.py

@@ -318,8 +318,6 @@ def _parse_args(args, **kwargs):
         if ns.use_mp <= 0:
             # Use all cores + extras for tests that like to sleep
             ns.use_mp = 2 + (os.cpu_count() or 1)
-        if ns.use_mp == 1:
-            ns.use_mp = None
     if ns.use:
         for a in ns.use:
             for r in a:

Misc/NEWS

@@ -22,6 +22,16 @@ Library
 - Issue #27056: Optimize pickle.load() and pickle.loads(), up to 10% faster
   to deserialize a lot of small objects.
 
+Tests
+-----
+
+- Issue #25285: regrtest now uses subprocesses when the -j1 command line option
+  is used: each test file runs in a fresh child process. Before, the -j1 option
+  was ignored.
+
+- Issue #25285: Tools/buildbot/test.bat script now uses -j1 by default to run
+  each test file in fresh child process.
+
 
 What's New in Python 3.6.0 alpha 1?
 ===================================

Tools/buildbot/test.bat

@@ -4,7 +4,7 @@ setlocal
 
 set here=%~dp0
 set rt_opts=-q -d
-set regrtest_args=
+set regrtest_args=-j1
 
 :CheckOpts
 if "%1"=="-x64" (set rt_opts=%rt_opts% %1) & shift & goto CheckOpts