test: make source-tests relative to ./test/data/

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.

test/data/sources/org.osbuild.files/cases/empty.json

@@ -0,0 +1,4 @@
+{
+  "expects": "success",
+  "checksums": []
+}