fix up test_loadstore1.py

fix up test_loadstore1.py

numerous things:
1) pi_ld and pi_st now return the exception information (full status)
   because pi_ld and pi_st both reset the PortInterface: once that
   happens the exception information is DESTROYED... therefore the
   exception information MUST be obtained INSIDE pi_ld and pi_st

2) cleanup of whitespace

3) added some (probably unnecessary) yields in between LD/ST in the
   exceptions test, to make the gtkwave output clearer

4) fixed the exceptions test and enabled it by default, now