miltertest: Fix broken mt.data() function

[glts]

The implementation of mt.data() is broken. The state management uses the
wrong constants, thereby causing any use of mt.data() to time out:

miltertest: select(): timeout on fd 3

The fix is straightforward. Align the mistaken use of STATE_* constants
in mt_data and mt_header with how it’s done elsewhere.

[glts]

I want to point out that miltertest cannot currently be used to test
milters that use the data stage due to this bug. This impacts the
usefulness of miltertest considerably.

The fix is trivial, so it would be great if you could find the time to
merge this.

[martinbogo]

Looks good, doesn't break the tests, so merging.

[glts]

Thanks for the merge!