run-tests: on windows, put correct python at front of PATH

The older approach of trying to copy the python executable into the test
directory was doomed to fail.

There remains one weakness with this approach: if you've run "make local",
tests may pick up the wrong extension DLLs from inside the source tree. I
don't know why this happens.

A reasonable workaround for now is to test either using --local or with
a working directory that does not contain built DLLs.

tests/run-tests.py

@@ -328,7 +328,7 b' def checktools():'
     # Before we go any further, check for pre-requisite tools
     # stuff from coreutils (cat, rm, etc) are not tested
     for p in requiredtools:
-        if os.name == 'nt':
+        if os.name == 'nt' and not p.endswith('.exe'):
             p += '.exe'
         found = findprogram(p)
         if found:
@@ -365,18 +365,24 b' def usecorrectpython():'
         exename = 'python'
         if sys.platform == 'win32':
             exename = 'python.exe'
-    vlog('# Making python executable in test path use correct Python')
+    if getattr(os, 'symlink', None):
+        vlog("# Making python executable in test path a symlink to '%s'" % 
+             sys.executable)
     mypython = os.path.join(BINDIR, exename)
     try:
         os.symlink(sys.executable, mypython)
-    except AttributeError:
-        # windows fallback
-        shutil.copyfile(sys.executable, mypython)
-        shutil.copymode(sys.executable, mypython)
     except OSError, err:
         # child processes may race, which is harmless
         if err.errno != errno.EEXIST:
             raise
+    else:
+        vlog("# Modifying search path to find %s in '%s'" % (exename, exedir))
+        path = os.environ['PATH'].split(os.pathsep)
+        while exedir in path:
+            path.remove(exedir)
+        os.environ['PATH'] = os.pathsep.join([exedir] + path)
+        if not findprogram(exename):
+            print "WARNING: Cannot find %s in search path" % exename
 
 def installhg(options):
     vlog("# Performing temporary installation of HG")