pandoc-3.10.1: test/command/1390.md

```
% pandoc -f latex -t native
\newcommand\foo{+}
Testing: $\mu\foo\eta$.
^D
[ Para
    [ Str "Testing:"
    , Space
    , Math InlineMath "\\mu+\\eta"
    , Str "."
    ]
]
```

<!-- It would be nice to handle this case, but I don't
know how:

```
% pandoc -f latex -t native
\newcommand{\vecx}{a + b}
$\hat\vecx$
^D
[Para [Math InlineMath "\\hat{a+b}"]]
```
-->