Fix weak-vtables warning

[nlohmann]

Fixes #4087

[coveralls]

coverage: 100.0%. remained the same
when pulling f1dd312 on fix-weak-vtables
into 64f68dc on develop.

[gregmarr]

I don't believe that it is worth trying to "fix" this. More details at #4087 (comment)

[nlohmann]

I forgot to also remove the previous fixes in the test cases and add a pragma.

[gregmarr]

Do we need to worry about someone building the tests with different warning flags? I would hope that we'd be able to say "the tests are built with exactly these flags".

[nlohmann]

Do we need to worry about someone building the tests with different warning flags? I would hope that we'd be able to say "the tests are built with exactly these flags".

In the end, I did not suppress the warning, but could fix it by completing the rule of five.