This moves the `sources_tests` into ./test/data/sources/ and makes the
entire test use `locate_test_data()` to get relative paths for their
accesses.
This further improves our test cases to support running from
installments rather than local checkouts. We need access to ./test/data
guarded, so we can install packages and still have the tests access the
correct paths.
This also adjusts the HTTP-Server we use in the test to serve data
relative to a path it is handed. I now chose `./test/data`, which will
easily allow us to re-use the same HTTP-Server in the future for other
tests that require it.
6e8562f712