gdb/testsuite: refactor gdb.threads/detach-step-over.exp

Factor out some bits of gdb.threads/detach-step-over.exp to procs in
preparation to adding some new variations of the test.  Rename the
existing "test" proc and make it use proc_with_prefix.

Co-Authored-By: Simon Marchi <simon.marchi@efficios.com>
Change-Id: Ib4412545c81c8556029e0f7bfa9dd48d7a9f3189
1 file changed