Add simple test for Balance Monitor

The test checks calls to the balance source function. In the future we
should check logged messages.

pkg/chain/ethlike/balance_monitor_test.go

@@ -0,0 +1,59 @@
+package ethlike
+
+import (
+	"context"
+	"fmt"
+	"math/big"
+	"sync"
+	"time"
+
+	"testing"
+
+	"github.com/ipfs/go-log"
+)
+
+func TestBalanceMonitor_Retries(t *testing.T) {
+	log.SetDebugLogging()
+
+	attemptsCount := 0
+	expectedAttempts := 3
+
+	wg := &sync.WaitGroup{}
+	wg.Add(expectedAttempts)
+
+	balanceSource := func(address Address) (*Token, error) {
+		attemptsCount++
+		wg.Done()
+
+		if attemptsCount < expectedAttempts {
+			return nil, fmt.Errorf("not this time")
+		}
+
+		return &Token{big.NewInt(10)}, nil
+	}
+
+	balanceMonitor := NewBalanceMonitor(balanceSource)
+
+	address := Address{1, 2}
+	alertThreshold := &Token{big.NewInt(15)}
+	tick := 1 * time.Minute
+	retryTimeout := 5 * time.Second
+
+	balanceMonitor.Observe(
+		context.Background(),
+		address,
+		alertThreshold,
+		tick,
+		retryTimeout,
+	)
+
+	wg.Wait()
+
+	if expectedAttempts != attemptsCount {
+		t.Errorf(
+			"unexpected retries count\nexpected: %d\nactual:   %d",
+			expectedAttempts,
+			attemptsCount,
+		)
+	}
+}