poly: add specific unit tests and simplify `DensePolynomial` addition… #2986

Job	Run time
Check (stable)	12s
Check (nightly)	1m 9s
Check Style	7s
Check Documentation	55s
Test (stable)	9m 0s
Test (nightly)	17m 29s
Check no_std	25s
List directories for parallelizing tests	3s
Test assembly	8m 19s
Test (cp6_782/)	5s
Test (mnt4_753/)	11s
Test (mnt6_753/)	11s
Test (./ed_on_mnt4_753)	3m 51s
Test (./secp256r1)	1m 0s
Test (./ed_on_mnt4_298)	1m 5s
Test (./ed_on_cp6_782)	1m 18s
Test (./mnt4_298)	3m 52s
Test (./mnt6_298)	5m 26s
Test (./ed_on_bn254)	1m 7s
Test (./bn254)	2m 13s
Test (./secp256k1)	1m 4s
Test (./bw6_761)	4m 3s
Test (./bls12_377)	4m 10s
Test (./grumpkin)	1m 9s
Test (./secp384r1)	1m 29s
Test (./ed_on_bls12_377)	1m 5s
Test (./secq256k1)	56s
Test (./ed25519)	1m 5s
Test (./ed_on_bls12_381)	1m 24s
Test (./ed_on_bw6_761)	43s
Test (./curve25519)	1m 11s
Test (./ed_on_bls12_381_bandersnatch)	1m 2s
Test (./pallas)	1m 4s
Test (./vesta)	1m 17s
Test (./bw6_767)	4m 9s
Test (./bls12_381)	3m 14s
	1h 27m 3s

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