tests/converter: modify unpack testcase

[SToPire]

This patch modifies the unpack testcase to keep consistent with the new nydus-image unpack interface in nydus-image.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 17 lines in your changes missing coverage. Please review.

Project coverage is 22.04%. Comparing base (067f6b2) to head (6964102). Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/converter/tool/builder.go	0.00%	17 Missing :warning:

Additional details and impacted files

[](https://app.codecov.io/gh/containerd/nydus-snapshotter/pull/614?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=containerd) ```diff @@ Coverage Diff @@ ## main #614 +/- ## ========================================== - Coverage 22.07% 22.04% -0.04% ========================================== Files 122 122 Lines 10768 10784 +16 ========================================== Hits 2377 2377 - Misses 8070 8086 +16 Partials 321 321 ``` | [Files with missing lines](https://app.codecov.io/gh/containerd/nydus-snapshotter/pull/614?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=containerd) | Coverage Δ | | |---|---|---| | [pkg/converter/tool/builder.go](https://app.codecov.io/gh/containerd/nydus-snapshotter/pull/614?src=pr&el=tree&filepath=pkg%2Fconverter%2Ftool%2Fbuilder.go&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=containerd#diff-cGtnL2NvbnZlcnRlci90b29sL2J1aWxkZXIuZ28=) | `0.00% <0.00%> (ø)` | |

[SToPire]

The smoke test should work properly if this PR is applied.

[SToPire]

@imeoer The unpack testcase should pass as expected with this PR merged.