[gdb/testsuite] Fix gdb.server/file-transfer.exp with local-remote-host.exp

[gdb/testsuite] Fix gdb.server/file-transfer.exp with local-remote-host.exp

When running test-case gdb.server/file-transfer.exp with host
board local-remote-host.exp, I get:
...
Executing on host: cmp -s $outputs/gdb.server/file-transfer/file-transfer \
  down-server    (timeout = 300)
builtin_spawn [open ...]^M
XYZ2ZYX
FAIL: gdb.server/file-transfer.exp: compare intermediate binary file
...

The remote host and remote target cases are handled here together here in proc
test_file_transfer:
...
    if {![is_remote host] && ![is_remote target]} {
       set up_server [standard_output_file $up_server]
       set down_server [standard_output_file $down_server]
    }
...

Fix this by handling them separately.

Tested on x86_64-linux.