test(SparseTerm): add small unit tests to cover impl #2967

Job	Run time
Check (stable)	9s
Check (nightly)	1m 12s
List directories for parallelizing tests	3s
Check Style	6s
Check Documentation	1m 3s
Test (stable)	8m 56s
Test (mnt4_753/)	18m 55s
Test (nightly)	17m 29s
Test (mnt6_753/)	12s
Check no_std	24s
Test assembly	8m 20s
Test (cp6_782/)	7s
Test (./ed_on_mnt4_753)	3m 48s
Test (./secp256r1)	54s
Test (./ed_on_mnt4_298)	1m 15s
Test (./ed_on_cp6_782)	1m 22s
Test (./mnt4_298)	3m 46s
Test (./mnt6_298)	5m 26s
Test (./ed_on_bn254)	57s
Test (./bn254)	2m 23s
Test (./secp256k1)	59s
Test (./bw6_761)	3m 44s
Test (./bls12_377)	4m 8s
Test (./grumpkin)	1m 6s
Test (./secp384r1)	1m 24s
Test (./ed_on_bls12_377)	1m 2s
Test (./secq256k1)	55s
Test (./ed25519)	54s
Test (./ed_on_bls12_381)	1m 13s
Test (./ed_on_bw6_761)	49s
Test (./curve25519)	1m 10s
Test (./ed_on_bls12_381_bandersnatch)	1m 0s
Test (./pallas)	1m 4s
Test (./vesta)	1m 6s
Test (./bw6_767)	4m 2s
Test (./bls12_381)	3m 11s
	1h 44m 34s

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