integration-test: Fix change label test with exfatprogs 1.2.5

Like VFAT, EXFAT does not allow some characters in the label.
Since version 1.2.5 exfatprogs refuses to set a label with those
invalid characters.
Because this changed in version 1.2.5 we cannot assert that setting
such label would throw without knowing the exfatprogs version.

While we're touching this part of the test, also document why we're
using a shorter label compared to the other filesystems.

src/tests/integration-test

@@ -1047,10 +1047,14 @@ class FS(UDisksTestCase):
  l = r'n"a$m\"e' + fs_type
  if fs_type == 'vfat':
  # VFAT does not support some characters
+ # Also, the maximum length of the label is 11 characters (22 bytes)
  self.assertRaises(GLib.GError, fs.call_set_label_sync, l, no_options, None)
  l = r'n@a$me'
  elif fs_type == 'exfat':
- l = r'n"a$m\"exft'
+ # EXFAT does not support some characters, but exfatprogs < 1.2.5 will still allow them.
+ # We cannot assert that the call would throw without knowing the exfatprogs version.
+ # Also, the maximum length of the label is 11 characters (22 bytes)
+ l = r'n@a$me'
  try:
  fs.call_set_label_sync(l, no_options, None)
  except GLib.GError as e: