Add debug = false to test-core.ini

This fixes a couple of tests (in the pdfpreview and jsonpreview
extensions) that would fail if you had debug = true in your
development.ini, because the tests look for the presence of minified
files that are only present with debug = false.

This seems like a good thing to do anyway, regardless of these failing
tests.
Merge pull request #458 from okfn/add-debug-equals-false-to-test-core-ini

Add debug = false to test-core.ini