Make blanket code coverage stats optional

[joeyparrish]

The code coverage stats added by blanket hinder debugging in the test runner, in particular by changing line numbers and hiding file names in the backtraces of test failures. It would be ideal if the usage of blanket was controlled by a URL parameter.

Not sure if the default should be on or off, but I'm leaning toward off. Code coverage stats are not used very often, but debugging a test failure is a fairly common operation.