Increased number of attempts for signing integration tests (#3725)

This PR increases the number of attempts for signing during integration
tests.
The reason for this change are occasional test failures.

pkg/tbtc/signing_test.go

@@ -185,7 +185,7 @@ func setupSigningExecutor(t *testing.T) *signingExecutor {
 
 	// Test block counter is much quicker than the real world one.
 	// Set more attempts to give more time for computations.
-	executor.signingAttemptsLimit *= 5
+	executor.signingAttemptsLimit *= 8
 
 	return executor
 }