tests: add missing :needs_network argument

I was recently running `brew tests` without `--online` and I noticed
that there was still one formulae.brew.sh request. I narrowed it down
to a `brew desc` test, so this adds `:needs_network` to that test.
As expected, `brew tests` doesn't make any network requests after this
change unless `--online` is used.

Library/Homebrew/test/cmd/desc_spec.rb

@@ -31,7 +31,7 @@ RSpec.describe Homebrew::Cmd::Desc do
       .and not_to_output.to_stderr
   end
 
-  it "successfully searches without --eval-all, with API", :integration_test do
+  it "successfully searches without --eval-all, with API", :integration_test, :needs_network do
     setup_test_formula "testball"
 
     expect { brew "desc", "--search", "testball", "HOMEBREW_NO_INSTALL_FROM_API" => nil }