fix: Elongate agent disconnect timeout in tests (#2687)

This will fix the flake seen here:
https://github.com/coder/coder/runs/7071719863?check_suite_focus=true

coderd/coderd.go

@@ -47,6 +47,7 @@ type Options struct {
 	CacheDir string
 
 	AgentConnectionUpdateFrequency time.Duration
+	AgentInactiveDisconnectTimeout time.Duration
 	// APIRateLimit is the minutely throughput rate limit per user or ip.
 	// Setting a rate limit <0 will disable the rate limiter across the entire
 	// app. Specific routes may have their own limiters.
@@ -69,6 +70,10 @@ func New(options *Options) *API {
 	if options.AgentConnectionUpdateFrequency == 0 {
 		options.AgentConnectionUpdateFrequency = 3 * time.Second
 	}
+	if options.AgentInactiveDisconnectTimeout == 0 {
+		// Multiply the update by two to allow for some lag-time.
+		options.AgentInactiveDisconnectTimeout = options.AgentConnectionUpdateFrequency * 2
+	}
 	if options.APIRateLimit == 0 {
 		options.APIRateLimit = 512
 	}