Add a KUnit test suite for the polynomial_calc() function, which had no
in-kernel test coverage.  The tests verify correct evaluation of constant,
linear, quadratic, and cubic polynomials, including negative coefficients,
negative input data, zero-coefficient terms.

The Kconfig entry uses 'select POLYNOMIAL' rather than 'depends on
POLYNOMIAL' because POLYNOMIAL is a promptless tristate that cannot
be manually enabled on UML without an explicit selector.

Link: https://lore.kernel.org/20260606030319.316752-1-adinata.softwareengineer@gmail.com
Signed-off-by: Adi Nata <adinata.softwareengineer@gmail.com>
Cc: Maxim Kaurkin <maxim.kaurkin@baikalelectronics.ru>
Cc: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Cc: David Gow <david@davidgow.net>
Cc: Rae Moar <raemoar63@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>