Tests "step in" functionality in debugger.

Debugger was enabled.
Set timer for test function.
Script execution paused.
Call stack:
    0) testFunction (debugger-step-in.html:14)
Stepping into...
Script execution resumed.
Script execution paused.
Call stack:
    0) testFunction (debugger-step-in.html:15)
Stepping into...
Script execution resumed.
Script execution paused.
Call stack:
    0) d (debugger-step-in.html:9)
    1) testFunction (debugger-step-in.html:15)
Script execution resumed.
Debugger was disabled.