refactor(tracex): convert to unit testing (#781)

The exercise already allowed me to notice issues such as fields not
being properly initialized by savers.

This is one of the last steps before moving tracex away from the
internal/netx package and into the internal package.

See https://github.com/ooni/probe/issues/2121

internal/engine/netx/tracex/event.go

@@ -1,5 +1,9 @@
 package tracex
 
+//
+// All the possible events
+//
+
 import (
 	"crypto/x509"
 	"net/http"