tests: client: windows: add various llb.AddMount related tests

[profnandaa]

After completing support for llb.AddMount in #5708 -- this made possible to add some more client integration tests that were making llb.AddMount calls. Most of the tests below are related to this:

• testCacheExportCacheKeyLoop
• testOCILayoutSource
• testBuildMultiMount
• testTarExporterSymlink
• testCacheExportIgnoreError
• testBasicCacheImportExport
• testUncompressedLocalCacheImportExport
• testImageManifestRegistryCacheImportExport
• testBasicLocalCacheImportExport
• testBasicInlineCacheImportExport
• testRegistryEmptyCacheExport
• testCachedMounts
• testDuplicateCacheMount
• testRunCacheWithMounts
• testMountWithNoSource
• testProxyEnv
• testRelativeMountpoint
• testValidateDigestOrigin
• testExportAnnotations
• testExportAnnotationsMediaTypes
• testAttestationDefaultSubject
• testAttestationBundle
• testLLBMountPerformance

Skipped tests (due to registry setup issue, to be addressed separately):

• testUncompressedRegistryCacheImportExport
• testBasicRegistryCacheImportExport
• testMultipleRegistryCacheImportExport

Related to #5678

[profnandaa]

Not seeing any major change spliting up client tests -- taking almost same time as previously. Investigating...

[tonistiigi]

Not seeing any major change spliting up client tests -- taking almost same time as previously. Investigating...

What was the timing? The split is based on all tests, so if a lot of them are still skipped, some of the splits may actually be empty. If this is case we could reorder tests or run them in different blocks as each Run() gets own slicing.

[profnandaa]

@tonistiigi - should be ready now, can take a look. I had included some fix for #5807 but I don't think it's comprehensive, working on it on a separate branch will send a PR.