Sanshikan rules are not seen as testable

[niclashedam]

Currently, Sanshikan rules are not determined to be testable, as they export a constant as an object with multiple arrow functions. Constants and arrow functions are by themselves not seen as testable, as constants are, well, constant and arrow functions can also be constant in some contexts (see fx. packages/alfa-aria/src/features/body.ts). Since Sanshikan rules should be tested, the isTestable heuristic needs to be extended.