Make shutdown timeout configurable for tests

TestWebauthnConfigChanges seems to need a longer shutdown timeout when running
on GitHub Actions. The problem manifests in errors like this:

```
2024-05-05T17:00:45.0503602Z     api_test.go:2919: TestWebauthnConfigChanges/Can_edit_GUIConfiguration.WebauthnUserId Put "http://127.0.0.1:37585/rest/config/gui": EOF []
2024-05-05T17:00:45.0566336Z --- FAIL: TestWebauthnConfigChanges/Can_edit_GUIConfiguration.WebauthnUserId (0.52s)
```

indicating that the server was forcefully shut down (or panicked, but that was
ruled out in these cases) before the request was fully written.

lib/api/api.go

@@ -93,6 +93,7 @@ type service struct {
 	listenerAddr         net.Addr
 	exitChan             chan *svcutil.FatalErr
 	miscDB               *db.NamespacedKV
+	shutdownTimeout      time.Duration
 
 	guiErrors logger.Recorder
 	systemLog logger.Recorder
@@ -130,6 +131,7 @@ func New(id protocol.DeviceID, cfg config.Wrapper, assetDir, tlsDefaultCommonNam
 		startedOnce:          make(chan struct{}),
 		exitChan:             make(chan *svcutil.FatalErr, 1),
 		miscDB:               miscDB,
+		shutdownTimeout:      100 * time.Millisecond,
 	}
 }
 
@@ -458,7 +460,7 @@ func (s *service) Serve(ctx context.Context) error {
 	}
 	// Give it a moment to shut down gracefully, e.g. if we are restarting
 	// due to a config change through the API, let that finish successfully.
-	timeout, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
+	timeout, cancel := context.WithTimeout(context.Background(), s.shutdownTimeout)
 	defer cancel()
 	if err := srv.Shutdown(timeout); err == timeout.Err() {
 		srv.Close()

lib/api/api_test.go

@@ -957,6 +957,10 @@ func TestApiCache(t *testing.T) {
 }
 
 func startHTTP(cfg config.Wrapper) (string, context.CancelFunc, error) {
+	return startHTTPWithShutdownTimeout(cfg, 0)
+}
+
+func startHTTPWithShutdownTimeout(cfg config.Wrapper, shutdownTimeout time.Duration) (string, context.CancelFunc, error) {
 	m := new(modelmocks.Model)
 	assetDir := "../../gui"
 	eventSub := new(eventmocks.BufferedSubscription)
@@ -984,6 +988,10 @@ func startHTTP(cfg config.Wrapper) (string, context.CancelFunc, error) {
 	svc := New(protocol.LocalDeviceID, cfg, assetDir, "syncthing", m, eventSub, diskEventSub, events.NoopLogger, discoverer, connections, urService, mockedSummary, errorLog, systemLog, false, kdb).(*service)
 	svc.started = addrChan
 
+	if shutdownTimeout > 0*time.Millisecond {
+		svc.shutdownTimeout = shutdownTimeout
+	}
+
 	// Actually start the API service
 	supervisor := suture.New("API test", suture.Spec{
 		PassThroughPanics: true,

lib/testutil/testutil.go

@@ -8,6 +8,7 @@ package testutil
 
 import (
 	"errors"
+	"os"
 	"sync"
 	"testing"
 
@@ -63,6 +64,14 @@ func (NoopCloser) Close() error {
 	return nil
 }
 
+func IfNotCI[T any](notCiValue T, ciValue T) T {
+	if os.Getenv("CI") == "true" {
+		return ciValue
+	} else {
+		return notCiValue
+	}
+}
+
 func ConcatSlices[T any](slices ...[]T) []T {
 	// TODO when go >= 1.22.0: Replace with slices.Concat
 	concatLen := 0