[gdb/testsuite] Add gdb.testsuite/board-sanity.exp

Add a test-case that tests the sanity of target/host boards.

It contains a number of tests related to remote file manipulation, exercising:
- remote_upload
- remote_download
- remote_file exists
- remote_file delete
which check that these work together as expected.

Tested on x86_64-linux, with all relevant gdb/testsuite/boards/*.exp boards.

For target board remote-stdio-gdbserver.exp, this revealed a trivial problem
with the return value of proc ${board}_file for delete, so fix this.

The test-case shows that the proc ${board}_download in
local-remote-host-native.exp is broken, so remove it.

Likewise for board local-remote-host.exp, so remove proc ${board}_download and
associated ${board}_file.

Tested on x86_64-linux.