regrtest: rename --slow option to --slowest

Thanks to optparse, --slow syntax still works ;-)
diff --git a/Lib/test/libregrtest/cmdline.py b/Lib/test/libregrtest/cmdline.py
index f2ec0bd..1f7aaed 100644
--- a/Lib/test/libregrtest/cmdline.py
+++ b/Lib/test/libregrtest/cmdline.py
@@ -168,7 +168,7 @@
                        help='display test output on failure')
     group.add_argument('-q', '--quiet', action='store_true',
                        help='no output unless one or more tests fail')
-    group.add_argument('-o', '--slow', action='store_true', dest='print_slow',
+    group.add_argument('-o', '--slowest', action='store_true', dest='print_slow',
                        help='print the slowest 10 tests')
     group.add_argument('--header', action='store_true',
                        help='print header with interpreter info')