lib/test-string_helpers: replace UNESCAPE_ANY by UNESCAPE_ALL_MASK

lib/test-string_helpers: replace UNESCAPE_ANY by UNESCAPE_ALL_MASK

When we get a random number to generate a flag in the valid range of
UNESCAPE flags, use UNESCAPE_ALL_MASK, It's more correct and prevents from
missed updates of the test coverage in the future if any.

Link: https://lkml.kernel.org/r/20230327142604.48213-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>