We were being inconsistent about how we tested whether dump flags
were enabled; in particular, sometimes we also checked the verbosity,
and sometimes we didn't.

This lead to oddities such as "ghc -v4" printing an "Asm code" section
which didn't contain any code, and "-v4" enabled some parts of
"-ddump-deriv" but not others.

Now all the tests use dopt, which also takes the verbosity into account
as appropriate.