sys-script: remove output directory if it exists

meson will occasionally call us even though the output directory
exists. Let's just nuke and recreate in that case.
diff --git a/test/create-sys-script.py b/test/create-sys-script.py
index 24a00f7..a4f1f30 100755
--- a/test/create-sys-script.py
+++ b/test/create-sys-script.py
@@ -36,6 +36,7 @@
 
 OUTFILE_FUNCS = r"""
 import os, sys
+import shutil
 
 def d(path, mode):
     os.mkdir(path, mode)
@@ -58,6 +59,8 @@
 
 os.chdir(sys.argv[1])
 
+if os.path.exists('sys'):
+    shutil.rmtree('sys')
 """