support/testing: test_lua_utf8: fix test after bump to lua-utf8 0.1.2

support/testing: test_lua_utf8: fix test after bump to lua-utf8 0.1.2

The commit 0f0649140f73645f4cd00c9c0d27c50e9fa1caf9 introduced a change
in the module name: utf8 -> lua-utf8.

The packages-file-list.txt show the change in the files intalled on the rootfs:

[lua-utf8 0.1.2-2 latest version]
lua-utf8,./usr/lib/luarocks/rocks-5.1/luautf8/0.1.2-2/doc/LICENSE
lua-utf8,./usr/lib/luarocks/rocks-5.1/luautf8/0.1.2-2/doc/README.md
lua-utf8,./usr/lib/luarocks/rocks-5.1/luautf8/0.1.2-2/luautf8-0.1.2-2.rockspec
lua-utf8,./usr/lib/luarocks/rocks-5.1/luautf8/0.1.2-2/rock_manifest
lua-utf8,./usr/lib/lua/5.1/lua-utf8.so

[lua-utf8 1.2-0 previous version]
lua-utf8,./usr/lib/luarocks/rocks-5.1/utf8/1.2-0/doc/README.md
lua-utf8,./usr/lib/luarocks/rocks-5.1/utf8/1.2-0/utf8-1.2-0.rockspec
lua-utf8,./usr/lib/luarocks/rocks-5.1/utf8/1.2-0/rock_manifest
lua-utf8,./usr/lib/lua/5.1/utf8.so

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/526036036

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>