Enable json1 extension by default

[lvivier]

ref #532

Enables the JSON1 extension added in 3.9.0, and adds a test to confirm the json() function exists.

[lvivier]

@Mithgol I've only added a simple test to confirm the existence of json(), modeled on the fts test. If you think we should do more, just say so.

[Mithgol]

Thanks for your contribution.

In a Travis CI job I've seen that the test passes successfully in “our” built SQLite but then fails when the shared (the local system's) SQLite is tested.

That's normal because that SQLite is not expected to have SQLITE_ENABLE_JSON1 enabled.

I'm going to mark the test as pending for now (fe4812e).

In the future we might want to skip this test only when the shared SQLite is tested (however, I don't know if Mocha permits this idea).

[Mithgol]

I guess I'd better try a change in the system's environment.

[Mithgol]

My changes (1fcb843) seem fine, the test now depends on the SQLite version (“our” or the system's) being tested.