[test] Reenable serialization test

!119 (merged) was intended to 'Remove a work-around for clang 5 and earlier'. In fact this was a work-around for newer clang versions. It added the test if the compiler was either not clang or not newer than clang 5. By removing these lines, the test was not enabled at all.

This patch reenables the test without the work-around. Hopefully the CI gained a newer boost version in the last 4 years, such that there's no need for the work-around.

This should finally fix #20 (closed).