Refactor gdb.trace/save-trace.exp

Refactor gdb.trace/save-trace.exp

Some code is duplicated, to run the test twice with absolute and
relative paths, so I factored it out in a few procs.  It uses
with_test_prefix to differentiate between test runs.

I replaced usages of "save-tracepoints" with "save tracepoint", since
the former is deprecated.

I also removed the "10.x", as it doesn't make much sense anymore.  It
isn't used in general in the testsuite, and I don't think it's really
useful.

gdb/testsuite/ChangeLog:

	* save-trace.exp: Factor out code to these...
	(gdb_save_tracepoints): New.
	(gdb_load_tracepoints): New.
	(do_save_load_test): New.