run-tests: work around the Windows firewall popup for server processes

Windows doesn't have a `python3` executable, so cc0b332ab9fc attempted to work
around the issue by copying the current python to `python3.exe`.  That put it in
`_tmpbindir` because of failures in `test-run-tests.t` when using `_bindir`,
which looked like a process was trying to open it to write out a copy while it
was in use.  (Interestingly, I couldn't reproduce this running the test by
itself in a loop for a couple of hours, but it happens constantly when running
all tests.)  The problem with using `_tmpbindir` is that it is the randomly
generated path for the test run, and instead of Windows Firewall remembering the
executable signature or image hash when allowing the process to open a server
port, it apparently remembers the image path.  That means every run will trigger
a popup to allow it, which is bad for firing off a test run and walking away.

I tried to symlink to the python executable, but that currently requires admin
priviledges[1].  This will prompt the first time if the underlying python binary
has never opened a server port, but appears to avoid it on subsequent runs.

[1] https://bugs.python.org/issue40687

Differential Revision: https://phab.mercurial-scm.org/D9815

.hgignore

@@ -26,6 +26,7 b' tests/.hypothesis'
 tests/hypothesis-generated
 tests/annotated
 tests/exceptions
+tests/python3
 tests/*.err
 tests/htmlcov
 build

tests/run-tests.py

@@ -3470,14 +3470,13 b' class TestRunner(object):'
                         raise
         else:
             # Windows doesn't have `python3.exe`, and MSYS cannot understand the
-            # reparse point with that name provided by Microsoft.  Copy the
-            # current interpreter to PATH with that name so the shebang lines
-            # work.
+            # reparse point with that name provided by Microsoft.  Create a
+            # simple script on PATH with that name that delegates to the py3
+            # launcher so the shebang lines work.
             if os.getenv('MSYSTEM'):
-                shutil.copy(
-                    sys.executable,
-                    _bytes2sys(self._tmpbindir + b'/python3.exe'),
-                )
+                with open(osenvironb[b'RUNTESTDIR'] + b'/python3', 'wb') as f:
+                    f.write(b'#!/bin/sh\n')
+                    f.write(b'py -3 "$@"\n')
 
             exedir, exename = os.path.split(sysexecutable)
             vlog(