Fix deletion of FinishBreakpoints

Fix deletion of FinishBreakpoints

Currently, FinishBreakpoints are set at the return address of a frame based on
the `finish' command, and are meant to be temporary breakpoints. However, they
are not being cleaned up after use, as reported in PR python/18655. This was
happening because the disposition of the breakpoint was not being set
correctly.

This commit fixes this issue by correctly setting the disposition in the
post-stop hook of the breakpoint. It also adds a test to ensure this feature
isn't regressed in the future.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=18655