fix(developer): support spaces in regression tests

Fixes #5216.

A `<text>` element in the regression text xml which contained only
whitespace would cause the import to crash as this would be ignored by
the XML parser, leading to a `Null` `nodeValue` for that element. To
solve this, we need to:

1. Use `xml:space="preserve"` attribute on `<text>` elements
2. Update the DTD to allow `xml:space` on `<text>` elements
3. While we are at it, move the DTD from `www.tavultesoft.com` to
   `api.keyman.com`.

See also:
* keymanapp/tavultesoft.com#4
* keymanapp/api.keyman.com#155