\input luaotfload.sty

\font\testd={name:amiri:script=arab;language=dflt;+anum}
\font\testa={name:amiri:script=arab;language=ara;+anum}
\font\testp={name:amiri:script=arab;language=far;+anum}
\font\tests={name:amiri:script=arab;language=snd;+anum}
\font\testu={name:amiri:script=arab;language=urd;+anum}
\font\testx={name:amiri:script=arab;language=xxx;+anum}

\def\test{\luatextextdir TRT ضرب 0123456789}
\testd \test\par
\testa \test\par
\testp \test\par
\tests \test\par
\testu \test\par
\testx \test\par
\bye