tests: log value of IsTerminal

Just log the result of `IsTerminal` in `TestTerminal`: ```console $ go test -v -run TestTerminal === RUN TestTerminal isatty_others_test.go:13: os.Stdout: true --- PASS: TestTerminal (0.00s) PASS ok github.com/mattn/go-isatty 0.160s $ go test -v -run TestTerminal | cat === RUN TestTerminal isatty_others_test.go:13: os.Stdout: false --- PASS: TestTerminal (0.00s) PASS ok github.com/mattn/go-isatty 0.203s ```