Use different StatsD port during testing (#1070)

Sometimes (especially on JRuby), the Puma plugin test fails. Use a
different port than the default, in the hopes it doesn't conflict with
the appsignal-agent process's StatsD server already using the port.

[skip changeset]

spec/lib/puma/appsignal_spec.rb

@@ -35,7 +35,7 @@ class StatsdServer
     def start
       stop
       @socket = UDPSocket.new
-      @socket.bind("127.0.0.1", 8125)
+      @socket.bind("127.0.0.1", ENV.fetch("APPSIGNAL_STATSD_PORT"))
 
       loop do
         # Listen for messages and track them on the messages Array.
@@ -115,13 +115,15 @@ def in_background(&block)
 
   def run_plugin(stats_data, plugin, &block)
     Puma._set_stats = stats_data
+    ENV["APPSIGNAL_STATSD_PORT"] = "8126"
     @statsd = StatsdServer.new
     @server_thread = Thread.new { @statsd.start }
     @server_thread.abort_on_exception = true
     @client_thread = Thread.new { start_plugin(plugin) }
     @client_thread.abort_on_exception = true
     wait_for(:puma_client_wait, &block)
   ensure
+    ENV["APPSIGNAL_STATSD_PORT"] = nil
     Puma._set_stats = nil
     # Stop all threads in test and stop listening on the UDPSocket
     @client_thread.kill if defined?(@client_thread) && @client_thread