[FAB-9492] Fix flaky discovery cache test

The discovery TestCachePurgeCache sometimes fails because of wrong
assumptions of partial cache purging that don't always hold:

When the test reaches line 184, the cache (capacity of 4) might have:
identity2
identity3
identity4
identity5

and in lines 185-195 we load the cache with:
identity1, identity2, identity3, identity4

Therefore, the following scenario might happen:

- identity1 is being put, so identity2 is evicted
- identity2 is being put, so identity3 is evicted
- identity3 is being put, so identity4 is evicted
- identity4 is being put, so identity5 is evicted

We got 4 evictions so far, in contrast to the (false) assumption
of having between 1 and 2.

This test changes the identities that are loaded initiality in the warmup
phase to 4 instead of 5,

and in the next step - we start with identity5 to be sure we evicted
some identity, and then only put 2 identities and we just check that
the whole cache wasn't purged.

Change-Id: If28ae7db4adbc6319d95a3d41e954d8b420c7876
Signed-off-by: yacovm <yacovm@il.ibm.com>

discovery/authcache_test.go

@@ -169,9 +169,8 @@ func TestCachePurgeCache(t *testing.T) {
 	cache := newAuthCache(as, authCacheConfig{maxCacheSize: 4, purgeRetentionRatio: 0.75})
 	as.On("ConfigSequence", "mychannel").Return(uint64(0))
 
-	// Warm up the cache - attempt to place 5 identities but the cache size is 4 so among the first 4 identities,
-	// only 3 of them should be cached
-	for _, id := range []string{"identity1", "identity2", "identity3", "identity4", "identity5"} {
+	// Warm up the cache - attempt to place 4 identities to fill it up
+	for _, id := range []string{"identity1", "identity2", "identity3", "identity4"} {
 		sd := common.SignedData{
 			Data:      []byte{1, 2, 3},
 			Identity:  []byte(id),
@@ -185,7 +184,7 @@ func TestCachePurgeCache(t *testing.T) {
 
 	// Now, ensure that at least 1 of the identities was evicted from the cache, but not all
 	var evicted int
-	for _, id := range []string{"identity1", "identity2", "identity3", "identity4"} {
+	for _, id := range []string{"identity5", "identity1", "identity2"} {
 		sd := common.SignedData{
 			Data:      []byte{1, 2, 3},
 			Identity:  []byte(id),
@@ -197,7 +196,7 @@ func TestCachePurgeCache(t *testing.T) {
 			evicted++
 		}
 	}
-	assert.True(t, evicted > 0 && evicted < 3)
+	assert.True(t, evicted > 0 && evicted < 4, "evicted: %d, but expected between 1 and 3 evictions", evicted)
 }
 
 func TestCacheConcurrentConfigUpdate(t *testing.T) {