Avoid creating a dummy chebfun (which is painfully slow).

Profiling `chebtest`I noticed that  a lot of time was spent in `tweakDomain()` creating a dummy zero `chebfun` with a given domain. (Presumably Matlab's slow OOP is to blame?)

In this pull request, I avoid creating a `chebfun` and just use a struct with the appropriate domain as a field. This reduces the time taken to run `chebtest('chebfun')` from 153s to 128s on my machine.

Note that I could only test a single run, due to #1103.