Improve gzip_zinfo unittest coverage

[djdongjin]

Is your feature request related to a problem? Please describe.

gzip_zinfo doesn't have any unit test, #287 will adds some initial unit tests for it after merge but we should try to add the missing unit tests.

• NewGzipZinfo: no positive testcases (can create GzipZinfo with valid data)
• NewGzipZinfoFromFile: no test.
• TestExtractDataFromBuffer: no positive testcases (can extract correct data from gzipzinfo)
• TestExtractData: same as above.
• remaining funcs: no test.

https://github.com/awslabs/soci-snapshotter/blob/main/compression/gzip_zinfo.go

Describe the solution you'd like

Improve test coverage.

[hanyuel]

The functions in gzip_zinfo.go are called solely in our ztoc/ code (specificallyztoc_builder.go and ztoc.go) which are already well covered by the tests inztoc_test.go. So I don't think adding more tests for gzip_zinfo.go would add much benefits. I think the priority for this ticket should be low.