Test Gem::Resolver::Specification#install
Default Resolver::Specification#install options

Now you don't need to provide an options hash which makes the tests look
a little nicer.
Alphabetize tests
Test Gem::Resolver::Specification#source
Support multiple sources in gem.deps.rb.lock

The GEMS section lists all sources for all gems and does not pin gems to
a particular source.  This was missing, so now we add all gems using
each source for lookup.

This commit is suspect.  If the dependency API is not available for a
source what happens?  See future commits.

See #851
Fix unused variable warning