Refrain from printing container string matches

[erik-inkapool]

_pure_is_inside_container prints all matches. Using test -n will just evaluate to true/false depending on if matches found, which seems like the intention of this tests. This fixes #166 for me, but I'm not sure if it's the only fix required to close that issue.

[edouard-lopez]

Thanks @erik-inkapool, I will have a look at this

[edouard-lopez]

Hello @erik-inkapool,
Thank you for your patience, I'm resuming my work on pure. I will first focus on fixing the tests, before merging the existing request that touch to the codebase, to ensure we have no regression.

[edouard-lopez]

@erik-inkapool I fixed issues related to container detection in https://github.com/pure-fish/pure/releases/tag/v4.2.2, https://github.com/pure-fish/pure/releases/tag/v4.2.3.
Is this issue still happening with version ≥4.4.0? If so, could you provide more context with:

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## Env

**Pure**

    echo $pure_version

**Fish**

    fish --version

**OS/distribution**

    lsb_release -a

Also share the output of your prompt if that's relevant.