add automated tests for svg.in-content.enabled
As a followup to legacy/trac#12827 (moved) and legacy/trac#15794 (moved), we should (as Mike suggested) add a bunch of tests for various SVG-related scenarios including things such as use of and <img> tags within <svg> blocks. Kathy and I tested quite a few scenarios when we did the work for legacy/trac#12827 (moved) but obviously we missed at least one.</p> <p data-sourcepos="3:1-3:81">I am opening this new, separate ticket so we do not lose track of this work item.</p>