[CSS Shapes] Move shape-inside floats tests into their own subdirectory

I'm going to add some new float tests soon. In order to keep our shapes tests organized,
I'm moving the shape-inside with floating content tests into their own subdirectory.

Review URL: https://codereview.chromium.org/26137003

git-svn-id: svn://svn.chromium.org/blink/trunk@159685 bbb929c8-8fbe-4397-9dbb-9b2b20218538