Use a private reference to JSON.parse to prevent it being mocked

PIE-979

It is possible for test suites to mock the parse method of JSON.
For example:

allow(JSON).to receive(:parse).and_raise(JSON::ParserError)

This will cause the test collector library to misbehave.

We can resolve this problem by updating the test collector code to
use a private reference to the JSON method which is immune to mocking.

This solution is similar to
#132
and https://github.com/ruby/timeout/pull/18/files

Resolves #144

lib/buildkite/test_collector/session.rb

@@ -7,6 +7,10 @@ class Session
     MAX_RECONNECTION_ATTEMPTS = ENV.fetch("BUILDKITE_ANALYTICS_RECONNECTION_ATTEMPTS") { 3 }.to_i
     WAIT_BETWEEN_RECONNECTIONS = ENV.fetch("BUILDKITE_ANALYTICS_RECONNECTION_WAIT") { 5 }.to_i
 
+    # We keep a private reference so that mocking libraries won't break JSON
+    JSON_PARSE = JSON.method(:parse)
+    private_constant :JSON_PARSE
+
     class RejectedSubscription < StandardError; end
     class InitialConnectionFailure < StandardError; end
 
@@ -120,7 +124,7 @@ def close(examples_count)
     end
 
     def handle(_connection, data)
-      data = JSON.parse(data)
+      data = JSON_PARSE.call(data)
       case data["type"]
       when "ping"
         # In absence of other message, the server sends us a ping every 3 seconds

spec/test_collector/session_spec.rb

@@ -53,6 +53,12 @@
       session.handle(socket_double, {"type"=> "message", "identifier"=> "fake_channel", "message" => {"confirm"=> ["./spec/analytics/session_spec.rb[1:1]"]}}.to_json)
       expect(session.unconfirmed_idents_count).to be 0
     end
+
+    it "is immune to JSON.parse being mocked" do
+      allow(JSON).to receive(:parse).and_raise(JSON::ParserError)
+
+      session.handle(socket_double, {"type"=> "message", "identifier"=> "fake_channel", "message" => {"confirm"=> ["./spec/analytics/session_spec.rb[1:1]"]}}.to_json)
+    end
   end
 
   describe "#close" do