Test the `http.Handler` returned from `zpages.NewTracezHandler`

[MrAlias]

There is currently zero coverage of this code. The added tests need to comprehensively cover the functionality provided.

[Oloruntobi1]

@MrAlias assign this to me?

[dmathieu]

This hasn't been updated in quite a while. Unassigning so someone else can pick it up (feel free to open a PR and link this issue in it, assignments are not mandatory).

[ayoisaiah]

I'm picking this up