feat: --grade-only flag to only test checks required for rating

[magnuslarsen]

Describe your changes

Fixes #2875

Added a very simple --grade-only flag, that will enable only the checks required for rating to be successful.

NOTE: this is a very simple implementation that sets the required variables to true (even if they potentially were disabled with other flags). HOWEVER it is also possible to add additional tests with any of the single-check flags (like ./testssl.sh --grade-only --winshock)

Not sure if this is too simple for your liking? Let me know if so, then I will update. I threw this together rather quickly :-)

...oh, and I lowercased the --disable-rating help text; all the surrounding text is lowercase

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

[drwetter]

Thanks a lot, @magnuslarsen !

A couple of points need some adjustments:

• I'd rather like to have this in 3.3dev first. Maybe it ends up in 3.2 later, but can't tell yet
• --grade-only [..] test only the checks required for rating should rather be in the tuning section
• $VULN_THRESHOLD needs to be re(-)adjusted , otherwise we'll get for each vulnerability a headline (that's legacy anyway, see remove underlined headline feature for each vulnerability #2943)
• man page (markdown) just needs a line (can do that myself)

Take your time let me know what you cannot do at the moment -- face the same situation though

[magnuslarsen]

Will fix this in a separate PR