[gdb/testsuite] Use require for ensure_gdb_index

Replace:
...
if { [ensure_gdb_index $binfile] == -1 } {
    return -1
}
...
with:
...
require {ensure_gdb_index $binfile} != -1
...
and consequently, add a missing UNTESTED message.

Tested on x86_64-linux, both with native and target board readnow.