Add support for EC private key in mTLS check

[24icewolf42]

Describe your changes

When testing mTLS with a client PEM including key and certificate, the testssl mtls check does not check for EC private keys.

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! Shouldn't this rather have been grep -Eq 'ARG1|ARG2' in the first place?

[drwetter]

Seems that expression slipped through before.

It works under Linux and MacOS (grep (BSD grep, GNU compatible) 2.6.0-FreeBSD) which I didn't think. But I'd like it to be in line with other regex grep commands here. Let me know if you rather want me to do that.

[24icewolf42]

So, I aligned the grep with the others.

[drwetter]

Thank you @24icewolf42 !