Run XML conformance tests under CI

Also add llvm to Docker image so that backtraces can be symbolized.
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 8b450b3..78784fb 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -6,15 +6,18 @@
   # RUN apt-get update && \
   #     apt-get upgrade -y && \
   #     apt-get install -y --no-install-recommends \
-  #         git ca-certificates \
+  #         curl git ca-certificates \
   #         autoconf automake libtool pkg-config \
-  #         make gcc clang \
+  #         make gcc clang llvm \
   #         python-dev zlib1g-dev liblzma-dev libgcrypt-dev
+  # WORKDIR /tests
+  # RUN curl https://www.w3.org/XML/Test/xmlts20080827.tar.gz |tar xz
   image: registry.gitlab.gnome.org/gnome/libxml2
 
 .test:
   script:
     - |
+      ln -s /tests/xmlconf
       sh autogen.sh $CONFIG
       make -j$(nproc) V=1 CFLAGS="$CFLAGS -Werror"
       make check