Small change to get `image dump separate-debug-info` working when using
`symbols.load-on-demand`.
Added tests to `TestDumpDwo`, and enabled the test for all platforms. If we fail to build, we skip the test, so this shouldn't cause the test to fail on unsupported platforms.
```
bin/lldb-dotest -p TestDumpDwo
```
It's easy to verify this manually by running
```
lldb --one-line-before-file "settings set symbols.load-on-demand true" <some_target>
(lldb) image dump separate-debug-info
...
```
---------
Co-authored-by: Tom Yang <toyang@fb.com>
69a5869da4