tests: Create separate self-signed EC key for tlsfuzzer testing

[Jakuje]

Description

Follow up from #488, where we found the tlsfuzzer has problems with heterogeneous CA + server keys. This creates separate EC key outside of the main CA chain (EC Peer key used for ECDH generally), which turned out to be easier than I expected. It also simplifies the setup a bit and removes the excluded test cases, that should be working.

Checklist

• Test suite updated with functionality tests
• Test suite updated with negative tests

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• This feature/change has adequate documentation added
• Code conform to coding style that today cannot yet be enforced via the check style test
• Commits have short titles and sensible commit messages
• Coverity Scan has run if needed (code PR) and no new defects were found

[simo5]

Only one nit, otherwise LGTM

[tomato42]

lgtm

[simo5]

LGTM as well