[DRE-maint] Bug#926381: ruby-fakeweb: FTBFS (failing tests)

[Lucas Nussbaum]

My understanding:
- ruby-fakeweb is a key package because ruby-thor build-depends on it
- ruby-fakeweb is unmaintained upstream
- ruby-thor doesn't actually need fakeweb
  + newer upstream releases use webmock instead:
    https://github.com/erikhuda/thor/commit/3241f2fbf1172b6182723b073fd4b390200660e9
  + ruby-fakeweb is only used for the test suite of ruby-thor, and
    ruby-thor does not include a test suite anymore (we should probably
    download from github rather than from rubygems.org, but that's
    another issue).

A solution that would work to make that bug disappear from the buster RC
bugs list is:
1) modify ruby-thor to drop the useless build-dependency on ruby-fakeweb
2) get it to migrate to testing
3) remove ruby-fakeweb from testing

Lucas