spec_helper: add check for unexpected network calls

Any test that is not tagged as :needs_network and that makes
a call to an unapproved method in the `Utils::Curl` module
will raise an error unless that method gets mocked somehow.

tests: add exceptions for tests that use curl to download local files

These are acceptable ways to use curl in local, non-network tests.
For those edge cases we allow you to bypass the check with :needs_utils_curl.

Library/Homebrew/test/cask/cask_spec.rb

@@ -46,13 +46,15 @@
       expect(c.token).to eq("caffeine")
     end
 
-    it "returns an instance of the Cask from a URL" do
+    it "returns an instance of the Cask from a URL", :needs_utils_curl do
+      expect(Homebrew::API).not_to receive(:fetch_json_api_file)
       c = Cask::CaskLoader.load("file://#{tap_path}/Casks/local-caffeine.rb")
       expect(c).to be_a(described_class)
       expect(c.token).to eq("local-caffeine")
     end
 
-    it "raises an error when failing to download a Cask from a URL" do
+    it "raises an error when failing to download a Cask from a URL", :needs_utils_curl do
+      expect(Homebrew::API).not_to receive(:fetch_json_api_file)
       expect do
         Cask::CaskLoader.load("file://#{tap_path}/Casks/notacask.rb")
       end.to raise_error(Cask::CaskUnavailableError)

Library/Homebrew/test/formulary_spec.rb

@@ -119,7 +119,8 @@ class Wrong#{described_class.class_s(formula_name)} < Formula
       expect(described_class.factory(formula_path)).to be_a(Formula)
     end
 
-    it "returns a Formula when given a URL" do
+    it "returns a Formula when given a URL", :needs_utils_curl do
+      expect(Homebrew::API).not_to receive(:fetch_json_api_file)
       formula = described_class.factory("file://#{formula_path}")
       expect(formula).to be_a(Formula)
     end

Library/Homebrew/test/spec_helper.rb

@@ -165,6 +165,15 @@
     skip "Requires network connection." unless ENV["HOMEBREW_TEST_ONLINE"]
   end
 
+  config.before do |example|
+    next if example.metadata.key?(:needs_network)
+    next if example.metadata.key?(:needs_utils_curl)
+
+    allow(Utils::Curl).to receive(:curl_executable).and_raise(<<~ERROR)
+      Unexpected call to Utils::Curl.curl_executable without setting :needs_network or :needs_utils_curl.
+    ERROR
+  end
+
   config.before(:each, :needs_svn) do
     svn_shim = HOMEBREW_SHIMS_PATH/"shared/svn"
     skip "Subversion is not installed." unless quiet_system svn_shim, "--version"