gdbtest: Fix one of the test methods

Make Breakpoint and BreakpointModel more robust.

Output:
QFATAL :
GDBDebugger::GdbTest::testInsertAndRemoveBreakpointWhileRunning()
ASSERT: "idx.isValid()" in file
/home/krf/devel/src/kdevplatform/debugger/breakpoint/breakpointmodel.cpp,
line 338