add tests

cmd/themis-server/main.go

@@ -540,7 +540,7 @@ func main() {
 		if len(absentees) == 0 {
 			msg = "Everybody can make it next Monday, see you then!"
 		} else {
-			msg = fmt.Sprintf("%s can't make it next Monday.", themis.FormatStringSlice(absentees))
+			msg = fmt.Sprintf("%s can't make it next Monday. :sad:", themis.FormatStringSlice(absentees))
 		}
 
 		_, err = discord.ChannelMessageSend(channelId, msg)

fmt_test.go

@@ -51,3 +51,44 @@ func TestFormatRowsInvalidQuery(t *testing.T) {
 	_, err = store.db.Query("SELECT count(name), distinct(trade_node) from provinces where region = 'France'")
 	assert.Error(t, err)
 }
+
+func TestFormatStringSlice(t *testing.T) {
+	tests := []struct {
+		name string
+		s    []string
+		want string
+	}{
+		{
+			name: "empty",
+			s:    []string{},
+			want: "",
+		},
+		{
+			name: "single",
+			s:    []string{"foo"},
+			want: "foo",
+		},
+		{
+			name: "two",
+			s:    []string{"foo", "bar"},
+			want: "foo and bar",
+		},
+		{
+			name: "three",
+			s:    []string{"foo", "bar", "baz"},
+			want: "foo, bar and baz",
+		},
+		{
+			name: "four",
+			s:    []string{"foo", "bar", "baz", "biz"},
+			want: "foo, bar, baz and biz",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := FormatStringSlice(tt.s); got != tt.want {
+				t.Errorf("FormatStringSlice() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}