Create alter column backend test fixture

Create alter column backend test fixture

For more expedient confirmation of op functionality,
start building generalized backend fixtures for ops.

Add basic round trip tests for alter column, obviously
disabled on SQLite.   Ensure this passes for the full
span of MySQL, Oracle, SQL Server on CI (PG is fine).
Next we can begin adding tests for all the MySQL issues
that are coming up such as #564, where regular ALTER COLUMN
is not consistently implemented based on datatypes etc.

Change-Id: I00ef174aa791ce7baeb764c5915591cd095deae0