gdb.base/warning.exp tweaks
    
    #1- Check that the warning is emitted.
    
    #2- Avoid overriding INTERNAL_GDBFLAGS, as per documentated in
        gdb/testsuite/README:
    
     ~~~
     The testsuite does not override a value provided by the user.
     ~~~
    
    We don't actually need to tweak INTERNAL_GDBFLAGS, we just need to
    append out -data-directory to GDBFLAGS, because each passed
    -data-directory option leads to a call to the warning:
    
     $ ./gdb -data-directory=foo -data-directory=bar
     Warning: foo: No such file or directory.
     Warning: bar: No such file or directory.
     [...]
    
    2018-11-19  Pedro Alves  <palves@redhat.com>
    
    	* gdb.base/warning.exp: Don't override INTERNAL_FLAGS.  Use
    	gdb_spawn_with_cmdline_opts instead of gdb_start.  Check that we
    	see the expected warning.