factor::tests: Check that powers of known-factorization numbers are factored correctly (#1831)
factor::tests: Check that powers of known-factorization numbers are factored correctly (#1831)

* factor::tests::recombines_factors: Minor refactor (skip useless bool)

* factor::tests: Check factorizations of powers of factored numbers

* factor::Factors: Add debug assertions to (Factor ^ Exponent)

* factor::tests: Drop obsoleted tests

`factor_correctly_recombines_prior_test_failures` was replaced with
`factor_2044854919485649` as this was the only test not subsumed.

* factor::tests::2044854919485649: Check the expected factorisation
uutils/coreutils,uutils/coreutils,uutils/coreutils