run-tests: open hgrc file only for writing

The file is not read here. Opening with "w+" is unnecessary.

tests/run-tests.py

@@ -333,7 +333,7 b' def findprogram(program):'
 
 def createhgrc(path, options):
     # create a fresh hgrc
-    hgrc = open(path, 'w+')
+    hgrc = open(path, 'w')
     hgrc.write('[ui]\n')
     hgrc.write('slash = True\n')
     hgrc.write('interactive = False\n')