test: fix variants of `TestAPIConformance/manifest_Put_Schema2_MissingConfigAndLayers` flake

What does this MR do?

Resolves #921 (closed)

The source of the flakiness were to small sizes of the layers generated by createRandomSmallLayer() function. This was causing collisions.

Depending on the layer size randomly chosen by the function, we were getting different likelihood for collision with layer size 0 being most likely. The test was pushing sample layer first and then it was generating two more layers that it was assuming will not be there in the repo, but in case of the collision, their sha was identical to the layer that was pushed just before and this way we were getting "not enough" missing layers error.

For example, the two flakes where we had good enough logging:

• https://gitlab.com/gitlab-org/container-registry/-/jobs/7848228739
• https://gitlab.com/gitlab-org/container-registry/-/jobs/7852705216

In both cases we can see two shas:

• 41538a8cd30a872df1ce8bf9a9e96f0231a3e926277cd51087adcf1c813fb7be - sha of the configuration blob
• e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 - sha of the 0-sized layer(!).

I have verified the behaviour using this simple program.

sha256:e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855: 500512
Slice sizes 0,
sha256:4bf5122f344554c53bde2ebb8cd2b7e3d1600ad631c385a5d7cce23c7785459a: 2053
Slice sizes 1,
sha256:7902699be42c8a8e46fbbb4501726517e86b22c56a189f7625a6da49081b2451: 2040
Slice sizes 1,
sha256:de5a6f78116eca62d7fc5ce159d23ae6b889b365a1739ad2cf36f925a140d0cc: 2039
Slice sizes 1,
sha256:04d6c0c946716aac894fc1653383543a91faab601302cf011607c82f06304651: 2038
Slice sizes 1,
sha256:83891d7fe85c33e52c8b4e5814c92fb6a3b9467299200538a6babaa8b452d879: 2037
Slice sizes 1,
sha256:3cbdaf66b3dd2b174788a2f17f938b52dda93a2a97440cead19332cbfacba7c8: 2037
Slice sizes 1,
sha256:8a8950f7623663222542c9469c73be3c4c81bbdf019e2c577590a61f2ce9a157: 2036
Slice sizes 1,
sha256:380918b946a526640a40df5dced6516794f3d97bbd9e6bb553d037c4439f31c3: 2033
Slice sizes 1,
sha256:1f184f101c67d585ba00d24cfde36863f747fbab9e84af7760ec771e9b36ca51: 2033
Slice sizes 1,
sha256:8a331fdde7032f33a71e1b2e257d80166e348e00fcb17914f48bdb57a1c63007: 2032
Slice sizes 1,
sha256:0a43b22d89fa2499be5c7704c9bf273260b0ca9588e4cd1897cd80f9c96cd97a: 2030
Slice sizes 1,
sha256:d4f09e5c5af99a24c7e304ca7997d26cb00901697de08a49be0d46ab5839b614: 2029
Slice sizes 1,
sha256:30a5bfa58e128af9e5a4955725d8ad26d4d574a537b58b7dc6d357acad578572: 2028
Slice sizes 1,
sha256:d1bbd73bb09190bfb883056771e22e997541ed20079793bf33975fe1654581c3: 2027
Slice sizes 1,
sha256:27abdeddfe8503496adeb623466caa47da5f63abd2bc6fa19f6cfcb73ecfed70: 2026
Slice sizes 1,
sha256:3e23e8160039594a33894f6564e1b1348bbd7a0088d42c4acb73eeaed59c009d: 2024
Slice sizes 1,
sha256:df7e70e5021544f4834bbee64a9e3789febc4be81470df629cad6ddb03320a5c: 2024
Slice sizes 1,
sha256:de7d1b721a1e0632b7cf04edf5032c8ecffa9f9a08492152b926f1a5a7e765d7: 2021
Slice sizes 1,
sha256:7da59d0dfbe21f43e842e8afb43e12a6445bbac07c2fc26984c71d0de3f99c9c: 2021
Slice sizes 1,
sha256:f4f97c88c409dcf3789b5b518da3f7d266c488066e97a606e38a150779880735: 2019
Slice sizes 1,
sha256:cbe5cfdf7c2118a9c3d78ef1d684f3afa089201352886449a06a6511cfef74a7: 2016
Slice sizes 1,
sha256:8d36bbb3d6fbf24f38ba020d9ceeef5d4562f5f26629f66b076ff395c438695e: 2015
Slice sizes 1,
sha256:956062137518b270d730d4753000896de17c100a42f9e24f5acee2faa75d5fdd: 2015
Slice sizes 1,
sha256:3973e022e93220f9212c18d0d0c543ae7c309e46640da93a4a0314de999f5112: 2013
Slice sizes 1,
sha256:27952171c7fcdf0ddc765ab4f4e1c537cb29e5e533d57b3456257ee785c81711: 2013
Slice sizes 1,
sha256:3ebe1b59762a1c8020c1efe3747dd07f0e30617ed60b4e6a5bee16b6ea421dd0: 2013
Slice sizes 1,
sha256:2d711642b726b04401627ca9fbac32f5c8530fb1903cc4db02258717921a4881: 2012
Slice sizes 1,
sha256:67c872d4912c71f15d2d6134ddf1d33d46f4bab2b56fe787522e7e4c9b58657d: 2007
Slice sizes 1,
sha256:d1211001882d2ce16a8553e449b6c8b7f71e61836efc2e416143808f20e721e7: 2006
Slice sizes 1,
sha256:591b7cc95037822dec5a4d593a2e2e8b19c07ddd2570e5699003d17f14c440a6: 2005
Slice sizes 1,
sha256:265fda17a34611b1533d8a281ff680dc5791b0ce0a11c25b35e11c8e75685509: 2005
Slice sizes 1,
sha256:72dfcfb0c470ac255cde83fb8fe38de8a128188e03ea5ba5b2a93adbea1062fa: 2003
Slice sizes 1,
sha256:cdb4ee2aea69cc6a83331bbe96dc2caa9a299d21329efb0336fc02a82e1839a8: 1999
Slice sizes 1,
sha256:e3b98a4da31a127d4bde6e43033f66ba274cab0eb7eb1c70ec41402bf6273dd8: 1997
Slice sizes 1,
sha256:35af2d15ebde4d67b29569ec19749e813c76f02286afc5a2a9871c2bfbdaab32: 1996

Once I have bumped the size of the layer to 128-196 range, I can no longer reproduce the collisions.

I have piggy-backed to this MR a small fix to the test I noticed while debugging this issue.

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