[tests] Add testing of keyboard events in FullscreenRenderer.GetChar()

This contains one test case of each tcell.Key* event type that can be
sent to and subsequently processed in fzf's GetChar(). The test cases
describe status quo, and all of them PASS.

Small function util.ToTty() was added. It is similar to util.IsTty(),
but for stdout (hence the To preposition).

src/tui/tcell.go

@@ -233,6 +233,7 @@ func (r *FullscreenRenderer) GetChar() Event {
 			return EventType(CtrlA.Int() - 'a' + int(r)).AsEvent()
 		}
 		switch ev.Key() {
+		// section 1: Ctrl+(Alt)+[a-z]
 		case tcell.KeyCtrlA:
 			return keyfn('a')
 		case tcell.KeyCtrlB:
@@ -285,6 +286,7 @@ func (r *FullscreenRenderer) GetChar() Event {
 			return keyfn('y')
 		case tcell.KeyCtrlZ:
 			return keyfn('z')
+		// section 2: Ctrl+[ \]_]
 		case tcell.KeyCtrlSpace:
 			return Event{CtrlSpace, 0, nil}
 		case tcell.KeyCtrlBackslash:
@@ -293,12 +295,14 @@ func (r *FullscreenRenderer) GetChar() Event {
 			return Event{CtrlRightBracket, 0, nil}
 		case tcell.KeyCtrlUnderscore:
 			return Event{CtrlSlash, 0, nil}
+		// section 3: (Alt)+Backspace2
 		case tcell.KeyBackspace2:
 			if alt {
 				return Event{AltBS, 0, nil}
 			}
 			return Event{BSpace, 0, nil}
 
+		// section 4: (Alt+Shift)+Key(Up|Down|Left|Right)
 		case tcell.KeyUp:
 			if altShift {
 				return Event{AltSUp, 0, nil}
@@ -344,6 +348,7 @@ func (r *FullscreenRenderer) GetChar() Event {
 			}
 			return Event{Right, 0, nil}
 
+		// section 5: (Insert|Home|Delete|End|PgUp|PgDn|BackTab|F1-F12)
 		case tcell.KeyInsert:
 			return Event{Insert, 0, nil}
 		case tcell.KeyHome:
@@ -356,10 +361,8 @@ func (r *FullscreenRenderer) GetChar() Event {
 			return Event{PgUp, 0, nil}
 		case tcell.KeyPgDn:
 			return Event{PgDn, 0, nil}
-
 		case tcell.KeyBacktab:
 			return Event{BTab, 0, nil}
-
 		case tcell.KeyF1:
 			return Event{F1, 0, nil}
 		case tcell.KeyF2:
@@ -385,6 +388,7 @@ func (r *FullscreenRenderer) GetChar() Event {
 		case tcell.KeyF12:
 			return Event{F12, 0, nil}
 
+		// section 6: (Alt)+'rune'
 		case tcell.KeyRune:
 			r := ev.Rune()
 			if alt {
@@ -392,12 +396,13 @@ func (r *FullscreenRenderer) GetChar() Event {
 			}
 			return Event{Rune, r, nil}
 
+		// section 7: Esc
 		case tcell.KeyEsc:
 			return Event{ESC, 0, nil}
-
 		}
 	}
 
+	// section 8: Invalid
 	return Event{Invalid, 0, nil}
 }