Update tests_latex.jl

Author: mortenpi

test/examples/tests_latex.jl

@@ -4,7 +4,7 @@ using Test
 # make.jl, and we need to set it to the relevant LaTeX builds.
 ENV["DOCUMENTER_TEST_EXAMPLES"] =
     "latex latex_simple latex_cover_page latex_toc_style latex_simple_tectonic " *
-    "latex_showcase"
+    "latex_showcase latex_showcase_listings"
 
 # When the file is run separately we need to include make.jl which actually builds
 # the docs and defines a few modules that are referred to in the docs. The make.jl