Add more tests for chebfun.norm().
Add more tests for chebfun.norm().

We now cover the 1 norm for scalar chebfuns and the 1, inf, and -inf norms for
array-valued ones.  The remaining norms that need tests are the p-norms for
both types and the 2-norm for array-valued chebfuns.

ilovejs/chebfun,alshedivat/chebfun,asgeirbirkis/chebfun