Pointers & errors 100% test coverage

[SoroushBeigi]

Pointers & errors chapter tests do not cover the whole codebase, because this function is not tested:

func (b Bitcoin) String() string {
	return fmt.Sprintf("%d BTC", b)
}

which can be covered with the test below:

t.Run("Bitcoin String", func(t *testing.T) {
		btc := Bitcoin(10)
		got := btc.String()
		want := "10 BTC"

		if got != want {
			t.Errorf("got %s want %s", got, want)
		}
	})

I strongly believe that we should encourage learners to find why the test coverage is not 100% and fix it as an assignment first.

[SoroushBeigi]

@quii should I go for it?

[quii]

Sure, but do a quick draft first so I can help you

[SoroushBeigi]

@quii I created this PR with basic practice exercise added. feel free to tell me any possible improvements