Added check for DHEater vulnerability and updated relevant tests (#211)

[BareqAZ]

Added a check for the DHEater vulnerability mentioned here: #211
And updated the tests. two things to note:
Considering this issue is in the algorithm not the server, the vulnerability was documented in the algorithm itself rather than the CVEs section.

And I did have to edit the following docker test to expect a failure instead of a success:
749: run_openssh_test "8.0p1" "test3" "${PROGRAM_RETVAL_FAILURE}"
I'm not sure if it's fine this way or if would you want to update the docker OpenSSH config instead.
Either way, I thought I'd mention these here to be sure.

[jtesta]

Since a much more thorough test for this vulnerability was implemented in 8190fe5, I'm closing this PR. Nevertheless, thank you @BareqAZ for putting in the effort! It is still very much appreciated!