[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/util/util.go

@@ -117,11 +117,16 @@ func DurWithin(
 	return val
 }
 
-// IsTty returns true is stdin is a terminal
+// IsTty returns true if stdin is a terminal
 func IsTty() bool {
 	return isatty.IsTerminal(os.Stdin.Fd())
 }
 
+// ToTty returns true if stdout is a terminal
+func ToTty() bool {
+	return isatty.IsTerminal(os.Stdout.Fd())
+}
+
 // Once returns a function that returns the specified boolean value only once
 func Once(nextResponse bool) func() bool {
 	state := nextResponse