Reduce --update invocations in tests

[niklasmohrin]

We have a lot of tests that initially run tldr --update. This is unnecessary as we could just place the needed pages in the temporary cache directory ourselves. This would a) speed up the tests, because they don't need to download the whole archive from the internet, b) remove unneeded internet traffic and c) make the tests reproducible and runnable in offline environments (like nix builds).

All tests not directly testing the update procedure should be changed.

[niklasmohrin]

Once #337 is merged, we can use it to set up our own update server when the tests are running. We should keep at least one test that actually updates from the official source, but it should be ignored by default