Fix race condition in tests for domains API cache

Description

This MR is my naive attempt to resolve race condition in tests for GitLab domains API cache.

See explanation below:

// Lookup function
func (c *client) GetLookup(ctx context.Context, _ string) api.Lookup {
	c.bootup <- c.stats.bumpStarted()
	defer c.stats.bumpLookups() // in the `cache` this code runs in a separate goroutine

	lookup := api.Lookup{}
	if c.failure == nil {
		lookup.Name = <-c.domain // once this unblocks tests, we immediately assert before we `bumpLookups`
	} else {
		lookup.Error = c.failure
	}

	return lookup
}

// Test:
t.Run("when item is in long cache only", func(t *testing.T) {
		withTestCache(resolverConfig{}, nil, func(cache *Cache, resolver *client) {
			cache.withTestEntry(entryConfig{expired: true, retrieved: true}, func(*Entry) {
				lookup := cache.Resolve(context.Background(), "my.gitlab.com")

				require.Equal(t, "my.gitlab.com", lookup.Name)
				require.Equal(t, uint64(0), resolver.stats.getLookups())

				resolver.domain <- "my.gitlab.com" // immediately after we read from the channel we continue to the assertion below
				require.Equal(t, uint64(1), resolver.stats.getLookups())
			})
		})
	})

Closes #436 (closed)