nit: use tab instead of space?

s/Register/Registration?

Maybe add some comment for this message like "PluginInfo is the message sent from a plugin to the Kubelet pluginwatcher for plugin registration".

Maybe add some comment for this message like "RegistrationStatus is the message sent from the Kubelet pluginwatcher to the plugin, indicating whether plugin registration succeeds (i.e., PluginRegistered is true) or the registration errors in case of failure".

s/v1beta2_field/v1beta1_field

s/device_plugin/plugin_test?

could you explain a bit why lines 103-107 are needed?

remove the two debugging logs?

same question here.

Similar question here. Is 1 sec enough? Mostly just personal preference, I usually prefer to just using "status = <-p.registrationStatus" in unit tests for simplicity, but if you feel using timeout bound wait is helpful, maybe we can define a helper func and use it consistently in this file?

Sure you can actually check for an error without starting a goroutine.
Usage of this function doesn't require you to start a waitgroup in the tests.

Code changes from:

var wg sync.WaitGroup
wg.Add(1)
go func() {
	require.NotNil(t, p.Serve(socketPath))
	wg.Done()
}()
c := make(chan struct{})
go func() {
	defer close(c)
	wg.Wait()
}()
select {
case <-c:
case <-time.After(time.Second):
	t.Fatalf("Timed out while on Serve()")
}

To:

require.NotNil(t, p.Serve(socketPath))
require.NotNil(t, p.Stop())

One last thing can we add a timeout on the context?
A one second timeout seems reasonable :)

nit: maybe add some comment explaining this field?

Looks like the comment about "hard-coded PluginsSockDir" is no longer true with some code refactoring. Maybe change the comment that the directory we watch is from kubelet.getPluginsDir() and also the rest of this file accordingly?