Test.extract: test that extracting outside of dest directory is an error

[StefanKarpinski]

We have always prevented this for security reasons, but although we have tested that the fancy attacks using symlinks are prevented, we haven't been testing that the basic attack of extracting a relative or absolute path outside of the tarball is prevented. This adds tests for that. It also factors the common logic for these attack tests into a helper function and tests that Tar.rewrite errors in the same way.

[codecov[bot]]

Codecov Report

Base: 97.30% // Head: 97.31% // Increases project coverage by +0.01% :tada:

Coverage data is based on head (63c5610) compared to base (1de4f92). Patch coverage: 100.00% of modified lines in pull request are covered.

:exclamation: Current head 63c5610 differs from pull request most recent head 564bdd2. Consider uploading reports for the commit 564bdd2 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #145 +/- ## ========================================== + Coverage 97.30% 97.31% +0.01% ========================================== Files 4 4 Lines 779 783 +4 ========================================== + Hits 758 762 +4 Misses 21 21 ``` | [Impacted Files](https://codecov.io/gh/JuliaIO/Tar.jl/pull/145?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO) | Coverage Δ | | |---|---|---| | [src/extract.jl](https://codecov.io/gh/JuliaIO/Tar.jl/pull/145/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO#diff-c3JjL2V4dHJhY3Quamw=) | `98.14% <100.00%> (+0.01%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaIO)

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