update assets paths for tests

js/components/playback.js

js/reveal.js

test/examples/auto-animate.html

test/examples/barebones.html

test/examples/embedded-media.html

test/examples/math.html

test/examples/slide-backgrounds.html

test/examples/slide-transitions.html

test/test-auto-animate.html

test/test-dependencies-async.html

test/test-dependencies.html

test/test-grid-navigation.html

test/test-iframe-backgrounds.html

test/test-iframes.html

test/test-markdown-element-attributes.html

test/test-markdown-external.html

test/test-markdown-options.html

test/test-markdown-slide-attributes.html

test/test-markdown.html

test/test-pdf.html