chore: improve link parser and its tests a bit

[Nerixyz]

• Adds a few more tests
• Switches to QCompareCaseInsensitive and std::set (no more allocations when searching for domains)

That's the most coverage we can get out of it (codecov doesn't really show which cases were covered in case of partial coverage).

None of the following checks will trigger, because there will always be three dots and never duplicated dots (guaranteed from the caller, parse):

chatterino2/src/common/LinkParser.cpp

Line 64 in a0b70b8

if (lastWasDot || octetNumber == 3)

I could remove them, but that makes it a bit more fragile.

Similar things go for

chatterino2/src/common/LinkParser.cpp

Line 89 in a0b70b8

return octetNumber == 3 && !lastWasDot;

With the only difference that we're not guaranteed that host doesn't end in . (there's a test for it).