fix(slh-dsa): use checked_shl for idx_tree range in tests#1224

tarcieri merged 2 commits intoRustCrypto:masterRustCrypto/signatures:masterfrom

sashass1315:fix/slh-dsa-test-idx-tree-range-256fsashass1315/signatures:fix/slh-dsa-test-idx-tree-range-256fCopy head branch name to clipboard