testutil: enable dev mode using a config flag.

This flag is not normally supported in Nomad, but we really need
to use it for testing to lower the timing values for registration
retries et. al. Instead of just enabling it on all tests, we
provide a bool flag in the config, which just makes the test
server pass the extra CLI arg when true.

testutil/server.go

@@ -37,6 +37,7 @@ type TestServerConfig struct {
 	Ports             *PortsConfig  `json:"ports,omitempty"`
 	Server            *ServerConfig `json:"server,omitempty"`
 	Client            *ClientConfig `json:"client,omitempty"`
+	DevMode           bool          `json:"-"`
 	Stdout, Stderr    io.Writer     `json:"-"`
 }
 
@@ -142,8 +143,13 @@ func NewTestServer(t *testing.T, cb ServerConfigCallback) *TestServer {
 		stderr = nomadConfig.Stderr
 	}
 
+	args := []string{"agent", "-config", configFile.Name()}
+	if nomadConfig.DevMode {
+		args = append(args, "-dev")
+	}
+
 	// Start the server
-	cmd := exec.Command("nomad", "agent", "-dev", "-config", configFile.Name())
+	cmd := exec.Command("nomad", args...)
 	cmd.Stdout = stdout
 	cmd.Stderr = stderr
 	if err := cmd.Start(); err != nil {