lib/tests: string_helpers: decouple unescape and escape cases

lib/tests: string_helpers: decouple unescape and escape cases

Patch series "lib/tests: string_helpers: Slight improvements".

Two ad-hoc patches to improve the test module.  It was induced by another
patch that poorly tried to add (existing) test cases and make me revisit
string_helpers_kunit.c.

This patch (of 2):

Currently the escape and unescape test cases go in one step.  Decouple
them for the better granularity and understanding test coverage in the
results.

Link: https://lore.kernel.org/20260406193425.1534197-1-andriy.shevchenko@linux.intel.com
Link: https://lore.kernel.org/20260406193425.1534197-2-andriy.shevchenko@linux.intel.com
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Kees Cook <kees@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>