feat(head): add TCs for HEAD on the Attribute, the Attribute value and /info/sourceIdentity (061_03, 061_04)

New Test Cases for HEAD on the Attribute, the Attribute value and the context source identity (TS 104-176 clause 5, 7.5.3.1, 7.6.3.1, 12.2.3.1). Stacked on the HEAD MR !337 (merged) (feat/head-success, which holds 061_01 and 061_02). The GET of these three resources is tested by !327 (merged), !333 (merged) and !328 (merged).

• 061_03: 204, without Content-Length, on /entities/{entityId}/attrs/{attrId}, /entities/{entityId}/attrs/{attrId}/value and /info/sourceIdentity
• 061_04: the GET's 400 or 404 on /attrs/{attrId} and /attrs/{attrId}/value - the six cases of 062_03 and 062_04: entity id not a valid URI (400), entity not known (404), attribute not known (404)

9 Test Cases in TP/NGSI-LD/CommonBehaviours/CommonResponses/VerifyHEAD/, all tagged since_ts104_v1.1.1.

As in !337 (merged), what is verified on success is the 204 and the missing Content-Length; the HTTP client does not read a body after a HEAD response.

Doc unit tests 13/13, statisticsDocumentationData.py 0 of 591 failed, robocop format --check clean, no new unused test data. Against coraine: 9/9.

MR 8 of the plan: https://famzang.com/ttf/ngsi-ld-suite-plan.html