Fix tests that use illegal ../ paths

I came across an issue while working on !311 (closed) and !315 (closed) where some tests pass inputs to internal/serving/disk with path traversal. This is not allowed in real life: the gitlab-pages process will always chdir into the pages root.

This MR changes the tests involved to use the same trick we use elsewhere in the project, namely, to change the working directory of the test process.