FEATURE: attribute to disable the fake Apex call for client only tests

[stevebuik]

Some tests need to wait for the initial Apex callback to load the component under test. Greased adds it's own Apex call to the boxcar in order to wait the correct amount of time before starting.

Other tests don't need to wait for an Apex callback i.e. they are client only tests or they use Apex but not for the initial load. In this case, the Greased Apex call is slowing down the test run.

Since refresh is done so much, a small gain like this adds up to real time saved over a day.