gdb/testsuite: use gdb_test in gdb.base/list.exp

Use gdb_test instead of send_gdb + gdb_expect.  Use proc_with_prefix to
help with name uniqueness.

gdb/testsuite/ChangeLog:

	* gdb.base/list.exp: Replace send_gdb + gdb_expect with
	  gdb_test.  Use proc_with_prefix.

Change-Id: Ieee8fb2c80f596f60397fab7633773a7f8c8c879