HttpTestServerDecoderFilterTest.HeaderMerge tests too many things

`HttpTestServerDecoderFilterTest.HeaderMerge` has grown too large, and it would be good to split
it up in a couple of smaller tests. As it stands, it tests too many things as a single unit test, and we can
make things easier to understand and also produce more helpful test failures by splitting it up.

This came up in https://github.com/envoyproxy/nighthawk/pull/512/files/79b25083d665124d713f53c6794f9a76dba9df21#diff-f3f9ecf5caebc75351abab7a618f0405 but as it is pre-existing, tracking this separately.