Enhancement to run SQLite unit tests with a database file, rather than in-memory db

[jdetaeye]

In my application and its test suite I also run an external process to process some data in the database. When using the SQLite database engine I can't use the in-memory database that is: it's only visible inside the python process running the test...

For other database engines, one can use the TEST_DATABASE_NAME to choose the database to use for the tests, but no such override capability is possible with SQLite.

This patch corrects this "unfair" inbalance:

1. When no TEST_DATABASE_NAME is specified in the settings file, it defaults to ":memory:", ie current behavior
   
2. If a TEST_DATABASE_NAME setting is present and it's different from ":memory:" we try to delete the database file before running the tests. Similar as the other database engines, the user is prompted for confirmation.
   
3. When the tests have finished, the test database file is deleted.

Since this change is backwards-incompatible, a design decision is required on this ticket.

[jdetaeye]

patch

[Jacob]

Seems reasonable. Needs a patch to the docs of TEST_DATABASE_NAME, but otherwise looks OK.

[jdetaeye]

Documentation patch

[jdetaeye]

The documentation patch goes a little further than only documenting this settings change.

The documentation on the unit tests described the test database and its settings under the section Understanding the test output. I believe the text is cleared and easier to read when the test database is covered in a seperate section The test database.

Feel free to revise and update further...

[Malcolm Tredinnick]

(In [6929]) Split up the documentation of the test database and the test output.

Refs #6134.

[Malcolm Tredinnick]

(In [6930]) Fixed #6134 -- Allow an on-disk SQLite database to be used for tests, if required. Patch from jdetaeye.