Avoid warnings due to multiple calls to configure

I was seeing multiple warnings like the following in the spec output:

    W, [2019-08-07T11:42:43.494699 #48484]  WARN -- : Adapter already configured!

This was because in the Adapter::Darwin spec in the "running threads"
describe block, both `subject.configure` and `subject.start` were being
called and the latter calls the former.

By moving the call to `subject.configure` into the "configuration"
describe block, we can avoid this problem.

spec/lib/listen/adapter/darwin_spec.rb

@@ -89,10 +89,13 @@
       expectations.each do |dir, obj|
         allow(obj).to receive(:watch).with(dir.to_s, latency: 0.1)
       end
-      subject.configure
     end
 
     describe 'configuration' do
+      before do
+        subject.configure
+      end
+
       context 'with 1 directory' do
         let(:directories) { expectations.keys.map { |p| Pathname(p.to_s) } }