fdo#70807: Improve unit test using custom styles

As the bug talks about issues in custom styles, now they're tested too.

Change-Id: I5f727718ec3591e29c44c49750a7c7a0841a8c61
Reviewed-on: https://gerrit.libreoffice.org/7373
Reviewed-by: Marcos Souza <marcos.souza.org@gmail.com>
Tested-by: Marcos Souza <marcos.souza.org@gmail.com>