Database improperly closed in example code

os.unlink(flaskr.DATABASE) causes the actual application database to be
purged; whereas, I reckon, one wants the _test_ database to be removed.
So every time I ran it, the test passed, but ended up crashing the live
app for want of a valid database.

I avoided using the sample code in examples/flaskr thus far, as I chose
to type out code from the turorial docs. The actual example code looks
good - at least to my beginner's eye.
Merge pull request #676 from lambdadi/master

Database improperly closed in the test case within this tutorial