Add test showing that resuming after a suspend schedules no redraw

When lazygit is suspended with ctrl+z and brought back with fg, nothing
deliberately triggers a redraw. The screen only repaints because the UI
thread happens to have a flush pending from the suspend keybinding, and
that flush races the SIGCONT handler's resume; when it loses in the
right way, the terminal shows a blank screen until the next input event
arrives (#5309).

pkg/gocui/suspend_test.go

@@ -0,0 +1,27 @@
+package gocui
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestResumeSchedulesRedraw(t *testing.T) {
+	g := newTestGui(t)
+
+	assert.NoError(t, g.Suspend())
+	assert.NoError(t, g.Resume())
+
+	ev := GocuiEvent{Type: eventNone}
+	select {
+	case ev = <-g.gEvents:
+	case <-time.After(100 * time.Millisecond):
+	}
+
+	/* EXPECTED:
+	assert.Equal(t, eventResize, ev.Type,
+		"resuming must schedule a redraw; without one the screen stays blank until the next event arrives")
+	ACTUAL: */
+	assert.Equal(t, eventNone, ev.Type)
+}