[tests] improve testfile_path() to download files

[JanMarvin]

Previously all files were downloaded prior to testing. Now the download is triggered by testfile_path(). This allows restoring parallel tests.

[JanMarvin]

Could you have a look @olivroy ? I remembered an earlier complaint from your side.

• Tests with external files should run only if the file was downloaded and downloads should only be attempted if online
• On the CI the tests with external files should only run on release and for coverage
• I didn't want to make things overly complicated, no md5 file hashes etc. If some test file is broken (e.g. partially downloaded, we can skip it)

[olivroy]

Well, it's just that at first, the downloads were problematic..

As you said, it's probably fine as is..
I know that styler has a skip_if_parallel() that may be handy.

I will do more research on how other packages do this. the zip package creates temp dir for that too.

https://testthat.r-lib.org/articles/parallel.html the testthat vignette

[olivroy]

I tripped on this again while working on #799 (solved by downloading the zip file from gh, unzip, then copy the contents manually to tests/testthat/testfiles

[JanMarvin]

Yes, I plan to merge it, but wanted to look into the comment above. Since testthat hides all the annoying network stuff I need and I don't want to add curl as dependency, I needed to play around with it.

[JanMarvin]

At least the code looks now a little less confusing