gdb.base/warning.exp tweaks

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.
 [...]

gdb/ChangeLog
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.