Avoid garbage gem.deps.rb.lock from test

TestGemRequestSet#test_install_from_gemdeps_install_dir used a Tempfile
for the gem.deps.rb.  When the .lock file is created during installation
it creates a new file in Dir.tmpdir which is not deleted.

Now the test uses plain open inside the rubygems test temp directory
which will get cleaned up properly.

Fixes #729