tests: Add a test case for path traversal in a dirtree

tests: Add a test case for path traversal in a dirtree

I was reading about a recent security issue with both EMC and VMWare:
https://arstechnica.com/information-technology/2018/01/emc-vmware-security-bugs-throw-gasoline-on-cloud-security-fire/

It's a classic path traversal problem, and that made me think more about our
handling of this in libostree.  Fortunately of course, not being new to
this rodeo, long ago I *did* consider path traversal.  Inside the pull
code, we call `ot_util_filename_validate()`.  Also, `fsck` does this too.

I have further followups here, but let's add some test cases for this. I crafted
a repository with a `../` in a dirtree object by patching libostree to inject
it, and that's included as a tarball.

This patch covers the two cases where we do already have checks; pulling
via HTTP, and in `fsck`.

Closes: #1412
Approved by: jlebon