There were a few 6.1 migration compatibility fixes in [previous][1]
[commits][2]. Most importantly, those commits reorganized some of the
compatibility tests to ensure that the tests would run against every
Migration version. To continue the effort of improving test coverage for
Migration compatibility, this commit converts tests for create_table and
change_column setting the correct precision on datetime columns.

While the create_table tests all pass, the change_column test did not
pass for 7.0 versioned Migrations on sqlite. This was due to the sqlite
adapter not using new_column_definition to set the options on the new
column (new_column_definition is where precision: 6 gets set if no
precision is specified). This happens because columns can't be modified
in place in sqlite and instead the whole table must be recreated and the
data copied. Before this commit, change_column would use the options
of the existing column as a base and merge in the exact options (and
type) passed to change_column.

This commit changes the change_column method to replace the existing
column without using the existing options. This ensures that precision:
6 is set consistently across adapters when change_column is used to
create a datetime column.

(manually backported to 7-0-stable because it did not apply cleanly)

[1]: c2f838e8
[2]: 9b07b2d6