gdb/testsuite: add a clear test to py-breakpoint.exp

This patch adds a test case to try to clear an internal python
breakpoint using the clear command.

This was suggested by Pedro during a code review of the following
commit.

  commit a5c69b1e
  Date:   Sun Apr 17 15:09:46 2022 +0800

    gdb: fix using clear command to delete non-user breakpoints(PR cli/7161)

Tested on x86_64 openSUSE Tumbleweed.