Rename TestPairSearch and refactor it

1. Extract test setup code so that it is visible what exactly is tested.
2. Removed check of checklist because that is tested by call to
    getBestAvailableCandidatePair.
3. Use require.* instead of assert.*
4. Removed check for leaked routines and timer. Because the test is
    super simple.

agent_get_best_available_candidate_pair_test.go

@@ -0,0 +1,28 @@
+//go:build !js
+// +build !js
+
+package ice
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestNoBestAvailableCandidatePairAfterAgentConstruction(t *testing.T) {
+	agent := setupTest(t)
+
+	require.Nil(t, agent.getBestAvailableCandidatePair())
+
+	tearDownTest(t, agent)
+}
+
+func setupTest(t *testing.T) *Agent {
+	agent, err := NewAgent(&AgentConfig{})
+	require.NoError(t, err)
+	return agent
+}
+
+func tearDownTest(t *testing.T, agent *Agent) {
+	require.NoError(t, agent.Close())
+}