Add tests for skip expansion without initial context

[mielvds]

Fixes #82. It confirms expected behaviour when compacting with new context but without initial context.

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
lib/pyld
canon.py	218	4	98%	27, 180, 565–566
context_resolver.py	102	8	92%	70, 129, 143, 189, 195–197, 221
iri_resolver.py	141	3	98%	307–308, 318
jsonld.py	2440	167	93%	274–278, 359, 396, 401, 411, 428–443, 479–480, 522, 530, 550, 558–559, 667–672, 752–753, 768–769, 829–834, 859–860, 875–876, 886–887, 912–913, 954, 964, 970–974, 985–986, 1019, 1028, 1035, 1100, 1123, 1145–1147, 1157–1160, 1175, 1198, 1210–1213, 1305, 1317–1320, 1340, 1369, 1402, 1406, 1443, 1486, 1504–1507, 1516, 1561, 1664–1671, 1698–1700, 1726–1729, 1905, 1943, 2376, 2385–2392, 2484, 2511, 2534, 2543, 3059, 3112, 3115, 3178, 3263, 3297, 3305, 3583, 3588, 3590, 3639, 3777–3781, 3847, 3884, 3947, 4167–4168, 4182, 4470, 4604, 4638, 4667–4670, 4696–4697, 4880, 4944, 4971, 5045, 5047, 5065, 5163, 5212, 5444, 5446, 5485, 5487, 5496, 5635, 5763, 5850, 5970–5984, 6046, 6048, 6116, 6329, 6335–6339, 6378, 6381, 6471–6472, 6481
lib/pyld/documentloader
aiohttp.py	65	22	66%	28–38, 69, 75, 87, 99–107, 118–121, 138, 153–155
requests.py	41	5	88%	55, 69, 81–89
TOTAL	3193	209	93%

Tests	Skipped	Failures	Errors	Time
1676	52 💤	0 ❌	0 🔥	11.625s ⏱️