[gdb/testsuite] Fix gdb.python/py-breakpoint.exp timeouts

[gdb/testsuite] Fix gdb.python/py-breakpoint.exp timeouts

On powerpc64le-linux, I run into two timeouts:
...
FAIL: gdb.python/py-breakpoint.exp: test_watchpoints: \
  Test watchpoint write (timeout)
FAIL: gdb.python/py-breakpoint.exp: test_bkpt_internal: \
  Test watchpoint write (timeout)
...

In this case, hw watchpoints are not supported, and using sw watchpoints
is slow.

Most of the time is spent in handling a try-catch, which triggers a malloc.  I
think this bit is more relevant for the "catch throw" part of the test-case,
so fix the timeouts by setting the watchpoints after the try-catch.

Tested on x86_64-linux and powerpc64le-linux.