How to convert Roman into Arabic numeral

\expandafter\rmntonum{\test} expands to \rmntonum{\test} since \expandafter applies to { (skipping over \rmntonum, as the name implies). What you're after is \expandafter\rmntonum\expandafter{\test} in order to expand \test. You can update \rmntonum to do this by default:

7 corresponds to VII
7 should not be -1!

\documentclass{article}

\usepackage{etoolbox}

\def\test{VII}
\let\oldrmntonum\rmntonum
\renewcommand{\rmntonum}[1]{\expandafter\oldrmntonum\expandafter{#1}}

\begin{document}

\rmntonum{VII} corresponds to \test

\rmntonum{\test} should not be -1!

\end{document}

You could save some keystrokes

\let\oldrmntonum\rmntonum
\renewcommand{\rmntonum}{\expandafter\oldrmntonum\expandafter}

but it requires you to explicitly brace the argument to \rmntonum (rather than, say, \rmntonum\test). Of course, this is not a bad idea in general.

It's a matter of turning a kernel function into a user level command, with expl3:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewExpandableDocumentCommand{\romantoarabic}{m}
 {
  \int_from_roman:f { #1 }
 }
\cs_generate_variant:Nn \int_from_roman:n { f }
\ExplSyntaxOff

\begin{document}

\romantoarabic{VII}

\newcommand{\seven}{VII}

\romantoarabic{\seven}

\end{document}

This prints two 7’s.

You can even do

\romantoarabic\seven

(but I wouldn't recommend it).

If you are using LuaLaTeX, there's a slicker definition:

\documentclass{article}

\usepackage{etoolbox}

\let\etoolboxrmntonum\rmntonum
\renewcommand{\rmntonum}[1]{%
  \expanded{\noexpand\etoolboxrmntonum{#1}}%
}

\newcommand\test{VII}
\newcommand\another{X\test}

\begin{document}

\rmntonum{VII} corresponds to \test

\rmntonum{\test} should not be -1!

\rmntonum\test{} should not be -1!

\rmntonum{C\another} should not be -1!

\end{document}