dlt: Add IPv6 gateway and refactor cmake unittest

[minminlittleshrimp]

cmake: correct typo in setting up evironment

unittest: pgrep should detect the exact pattern

cmake: refactor wrong line continuation

unittest: adapt IPv6 testing

• Checking NULL before checking IP
• Add environment variable for default loopback IP
• Add test for IPv6 in unittest, by using converted IP
• Add correct IP protocol for testing to dlt_gateway.conf

[michael-methner]

Hello @minminlittleshrimp , i am fine with the changes but could you please add a proper commit message. It should describe what's in the patch not from whom it is coming.

[minminlittleshrimp]

Hello @minminlittleshrimp , i am fine with the changes but could you please add a proper commit message. It should describe what's in the patch not from whom it is coming.

Hello @michael-methner kindly review, I already edit the commit title. Thank you.