Add Unit Tests for User Space Convertor

[estebanreyl]

What this PR does / why we need it:
This PR introduces tests for multiple functions in the user space convertor. This is meant to add to the long-term reliability and maintainability of the project. Part of #198

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?

Notes:
As part of this change, I had to introduce registry mocking for resolve/push/fetch functionality, as a part of this I added the ORAS library which has a nice library to use for the in-storage implementation. I also added a few Manifests and layers as part of this. Tried to keep it minimal as this is checked in. Let me know if there are any concerns. Also, I am not the best with Unit tests so any feedback is greatly appreciated.

ChangeLog

Bugfix

• Added a small bugfix to builder to remove a small contention issue found while testing.

Tests

builder_engine.go

• uploadManifestAndConfig -> Added test
• getBuilderEngineBase -> Added test
• isGzipLayer -> Added Test

builder_utils.go

• fetch - Covered by fetchManifest and fetchConfig
• fetchManifest -> Added test
• fetchConfig -> Added test
• fetchManifestAndConfig -> Seemed unnecessary
• downloadLayer -> Added test
• writeConfig -> Added test
• getFileDesc -> Added test
• uploadBlob -> Added test
• uploadBytes -> Added test
• buildArchiveFromFiles -> Left For later
• addFileToArchive -> Left For later

builder.go

• build -> Added test

overlaybd_builder.go

• checkForConvertedLayer -> Added test
• storeConvertedLayer -> Added test
  I am not currenly sure how best to add the remaining functions due to their use of the overlaybd binaries. In the near term these should be covered by unit tests but leaving as a future work item.

fastoci_builder.go
Similar problem to above. Leaving for later.

End to End tests

• The existing CI seems like the best place to add end to end tests. Left for later.

Testing Tools

Local Remotes
This is an abstraction to interact with a local registry. The registry itself supports fetching, resolving, and pushes. See the local_registry.go file for more info. Along with this there is a mocks/registry folder which allows us to load stored images into the local registry for testing for now I've kept the added images small and restricted to hello-world but more can be added easily.

Filesystem interactivity
test_utils.go introduces RunTestWithTempDir which is a helper emulating the snapshotter tests that allows us to run a test with a temporary directory. This is useful for testing the filesystem interactions of several of the functions.

In Memory DB
In memory DB for testing DB related operations

[liulanzheng]

@WaberZhuang cc please

[BigVan]

Amazing...

[liulanzheng]

LGTM

[WaberZhuang]

LGTM