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

With test-case gdb.python/py-breakpoint.exp I run into:
...
(gdb) ERROR: tcl error sourcing gdb.python/py-breakpoint.exp.
ERROR: can't read "skip_hw_watchpoint_tests_p": no such variable
    while executing
"if {$skip_hw_watchpoint_tests_p} {
        gdb_test_no_output "set can-use-hw-watchpoints 0" ""
    }"
...

Fix this by adding the missing "global skip_hw_watchpoint_tests_p" in two
procs.

Tested on x86_64-linux.