test: add test files for g:vimtex_syntax_custom_envs

test/test-syntax/test-custom-envs.tex

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+\documentclass{minimal}
+
+\usepackage{listings}
+\lstnewenvironment{code}[1][]{\lstset{#1}}{}
+\lstnewenvironment{python_code}{\lstset{language=python}}{}
+
+\begin{document}
+
+\begin{MyMathEnv}
+  f(x) = 1
+\end{MyMathEnv}
+
+\begin{python_code}
+def test(argument: str) -> str:
+    """
+    Test function.
+    """
+    return argument
+\end{python_code}
+
+\begin{code}[language=rust]
+fn main() {
+    println!("Hello World!");
+}
+\end{code}
+
+\end{document}

test/test-syntax/test-custom-envs.vim

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+source common.vim
+
+let g:vimtex_syntax_custom_envs = [
+      \ {'name': 'MyMathEnv', 'mathmode': 1},
+      \ {'name': 'python_code', 'nested': 'python'},
+      \ {
+      \   'name': 'code',
+      \   'nested': {
+      \     'python': 'language=python',
+      \     'c': 'language=C',
+      \     'rust': 'language=rust',
+      \   },
+      \ },
+      \]
+
+Edit test-custom-envs.tex
+
+if empty($INMAKE) | finish | endif
+
+call vimtex#test#finished()