pluginmanager: WaitForFirstFingerprint times out (#9597)

As pointed out by @tgross[1], prior to this change we would have been blocking
until all managers waited for first fingerprint rather than timing out as
intended.

1: https://github.com/hashicorp/nomad/pull/9590#discussion_r539534906

client/pluginmanager/testing.go

@@ -1,10 +1,22 @@
 package pluginmanager
 
+import "context"
+
 type MockPluginManager struct {
-	RunF      func()
-	ShutdownF func()
+	RunF                      func()
+	ShutdownF                 func()
+	WaitForFirstFingerprintCh <-chan struct{}
 }
 
 func (m *MockPluginManager) Run()               { m.RunF() }
 func (m *MockPluginManager) Shutdown()          { m.ShutdownF() }
 func (m *MockPluginManager) PluginType() string { return "mock" }
+func (m *MockPluginManager) WaitForFirstFingerprint(ctx context.Context) <-chan struct{} {
+	if m.WaitForFirstFingerprintCh != nil {
+		return m.WaitForFirstFingerprintCh
+	}
+
+	ch := make(chan struct{})
+	close(ch)
+	return ch
+}