Fix missing issuer CN

[drwetter]

Describe your changes

This fixes a problem which was introduced @ 8d8f83a. It caused for some hosts not to parse / display the issuer CN correctly.

Also it adds some code in testssl.sh and in a unit test to detect this earlier. In general an output string FIXME will now cause a unit test to fail. This can + should be used at other places too!

Fixes #2789

Please refer to an issue here or describe the change thoroughly in your PR.

What is your pull request about?

• Bug fix
• Improvement
• New feature (adds functionality)
• Breaking change (bug fix, feature or improvement that would cause existing functionality to not work as expected)
• Typo fix
• Documentation update
• Update of other files

If it's a code change please check the boxes which are applicable

• For the main program: My edits contain no tabs, indentation is five spaces and any line endings do not contain any blank chars
• I've read CONTRIBUTING.md and Coding_Convention.md
• I have tested this fix or improvement against >=2 hosts and I couldn't spot a problem
• I have tested this new feature against >=2 hosts which show this feature and >=2 host which does not (in order to avoid side effects) . I couldn't spot a problem
• For the new feature I have made corresponding changes to the documentation and / or to help()
• If it's a bigger change: I added myself to CREDITS.md (alphabetical order) and the change to CHANGELOG.md