Re-enable StepOverWatchpoint test, make it more focused

This is my second recent change to TestStepOverWatchpoint.py,
the first was to change the two global variables it is watching
from 'char' to 'long' so they're more likely to be on separate
words/doublewords of memory that can be watched indepdently.

I believe this removes the need for the MIPS and S390X skips.

The test was testing a combination of read and write watchpoints,
stepping over a function that hits them, and then instruction stepping
over a source line which hits them.  But previously it was
always starting with the read watchpoint in both of them, it
didn't test the instruction-stepping for the read watchpoint.

I now have to tests in TestStepOverWatchpoint.py, one which
runs to the read-watchpoint function, sets that watchpoint,
steps over another function which reads from that global, then
instruction steps over a source line that reads from that global.

The second test runs to the write-watchpoint function, sets that
watchpoint.  Steps over another function which writes to that
global, then instruction steps over a source line that writes to
that global.

These are both xfailed on Darwin systems because watchpoints and
hardware breakpoints are currently disabled by the kernel when
instruction stepping.