Sort TLS extensions

[dcooper16]

Describe your changes

This PR modifies testssl.sh so that run_server_defaults() prints the server's supported TLS extensions sorted by extension number rather than listing them in the order in which they were found. This simplifies comparing the results of two tests of a server using different versions of OpenSSL, as the two tests may not detect all of the extensions in the same order.

In order to simplify the sorting of the extensions, this PR changes $TLS_EXTENSIONS from a string to an array. In February 2017 comments were added (925e106) saying that it would be $TLS_EXTENSIONS were an array. So, this PR addresses those comments. However, it is possible that the reason for those comments no longer apply.

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 and the indentation is five spaces
• 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

[drwetter]

Yeah, as I was reading #2686 yesterday, I was thinking "hmm, that could be put in the proper order" and same day you did it 👍