test: remove "Generate some messages" test from raw

Before the change, the first subtest in raw format tests just
generated messages and checked that they are added successfully.  This
is not really a raw format test, it is creating of environment
required for other subtests to run.  The patch removes the first
subtest from raw and replaces it with bare add_message calls, similar
to how it is done in other tests.

TODO: we should check that test environment was created successfully.
Currently, many tests do add_message(), notmuch new and other calls
without checking the results.  We should come up with a general
solution for this, i.e. if any command during test initialization
fails, all tests should be skipped with appropriate error message.