add test to verify admonition icon keys in theme are not flattened
add test to verify admonition icon keys in theme are not flattened

dsisnero/asciidoctor-pdf,asciidoctor/asciidoctor-pdf,Hextremist/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,dsisnero/asciidoctor-pdf,asciidoctor/asciidoctor-pdf,Hextremist/asciidoctor-pdf