Add acceptance test for custom makeindex styles

services/clsi/test/acceptance/fixtures/examples/makeindex-custom-style/main.tex

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+\documentclass{article}
+
+\usepackage{makeidx}
+\makeindex
+
+\begin{document}
+
+To solve various problems in Physics \index{Physics} it can useful to express any arbitrary piecewise-smooth function as a Fourier Series \index{Fourier Series} composed of multiple sine and cosine funcions.
+
+\printindex
+
+\end{document}

services/clsi/test/acceptance/fixtures/examples/makeindex-custom-style/style.ist

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+heading_prefix "{\\bfseries\\hfil "
+heading_suffix "\\hfil}\\nopagebreak\n" 
+headings_flag 1
+delim_0 "\\dotfill"
+delim_1 "\\dotfill"
+delim_2 "\\dotfill"
+