fix: Use URI.open to fix Kernel#open deprecation warning

Fixes deprecation warning message:
"/home/runner/work/rigse/rigse/rails/spec/support/solr_spec_helper.rb:23:
warning: calling URI.open via Kernel#open is deprecated, call URI.open
directly or use URI#open"

See: https://github.com/concord-consortium/rigse/actions/runs/5728882369/job/15530837575#step:17:9

rails/spec/support/solr_spec_helper.rb

@@ -1,3 +1,5 @@
+require 'open-uri'
+
 $original_sunspot_session = Sunspot.session
 Sunspot.session = Sunspot::Rails::StubSessionProxy.new($original_sunspot_session)
 
@@ -20,7 +22,7 @@ def reindex_all
   end
 
   def test_solr_server
-    open("http://#{SOLR_HOST}:#{SOLR_PORT}/")
+    URI.open("http://#{SOLR_HOST}:#{SOLR_PORT}/")
   end
 
   def solr_setup