[gdb/testsuite] Fix stray file in gdb.dwarf2/gdb-index-nodebug.exp

After running test-case gdb.dwarf2/gdb-index-nodebug.exp I have:
...
$ ls build/gdb/testsuite
cache       config.status                gdb.log  lib       outputs   site.exp
config.log  gdb-index-nodebug.gdb-index  gdb.sum  Makefile  site.bak  temp
...

The file gdb-index-nodebug.gdb-index doesn't belong there.

It happens to be there because we do:
...
set index_file ${testfile}.gdb-index
set cmd "save gdb-index [file dirname ${index_file}]"
...
which results in:
...
(gdb) save gdb-index .
...

The intention was possibly to use $binfile instead of $testfile, but using
that wouldn't work for remote host.

Fix this by using host_standard_output_file.

Tested on x86_64-linux.