[testDBTablesMonitoring] Create instances with a macro.

After DBTablesMonitoring inherits DBTables, we will have to change the
way to make an instance for the test. This macro helps to reduce
lines to be changed when the time comes.
Merge pull request #443 from project-hatohol/test-dbtables-user-monitoring

[testDBTablesMonitoring] Create instances with a macro.