Fix test suite failure in file-then-restart.exp

Simon pointed out that the new file-then-restart.exp test fails with
the extended-remote target board.

The problem is that the test suite doesn't use gdb_file_cmd -- which
handles things like "set remote exec-file".  This patch changes
gdb_file_cmd to make the "kill" command optional, and then switches
the test case to use it.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30933
Approved-By: Simon Marchi <simon.marchi@efficios.com>
3 files changed