feat(developer): LDML tool for users to run the LDML tests

Provide an end user feature for running the LDML regression tests.

```
            <keystroke key="s"/>
            <check result="abc\u0022...s" />
            <keystroke key="t"/>
            <check result="abc\u0022...st" />
```

At present this is in ldml.cpp / ldml_test_source.cpp