mi, testsuite: add another EXTRA option to mi_expect_stop
    
    Allow the EXTRA argument of mi_expect_stop to contain a third pattern that must
    occur directly before the GDB prompt at the end of the output.
    
    Signed-off-by: Markus Metzger  <markus.t.metzger@intel.com>
    
    testsuite/
    	* lib/mi-support.exp (mi_expect_stop): Add third EXTRA field.
    
    Change-Id: I7b492b72619265ea81200935adf22d94bb086806