tests/docker: avoid invalid escape in Python string

This is an error in Python 3.12; fix it by using a raw string literal.

Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tests/docker/docker.py

@@ -186,7 +186,7 @@ def _check_binfmt_misc(executable):
               (binary))
         return None, True
 
-    m = re.search("interpreter (\S+)\n", entry)
+    m = re.search(r"interpreter (\S+)\n", entry)
     interp = m.group(1)
     if interp and interp != executable:
         print("binfmt_misc for %s does not point to %s, using %s" %