[gdb/testsuite] Use save_vars to restore GDBFLAGS

There's a pattern of using:
...
set saved_gdbflags $GDBFLAGS
set GDBFLAGS "$GDBFLAGS ..."
<do something with GDBFLAGS>
set GDBFLAGS $saved_gdbflags
...

Simplify this by using save_vars:
...
save_vars { GDBFLAGS } {
    set GDBFLAGS "$GDBFLAGS ..."
    <do something with GDBFLAGS>
}
...

Tested on x86_64-linux.