Loki: Fix memchunk headblock filter (#8591)

**What this PR does / why we need it**:
Fix our memchunk headblock filter to not skip entries that have
timestamp=maxT.

pkg/chunkenc/memchunk.go

@@ -1005,7 +1005,10 @@ func (hb *headBlock) Iterator(ctx context.Context, direction logproto.Direction,
 	baseHash := pipeline.BaseLabels().Hash()
 	process := func(e entry) {
 		// apply time filtering
-		if e.t < mint || e.t >= maxt {
+		if e.t < mint || e.t > maxt {
+			return
+		}
+		if e.t == maxt && mint != maxt {
 			return
 		}
 		stats.AddHeadChunkBytes(int64(len(e.s)))

pkg/chunkenc/memchunk_test.go

@@ -911,8 +911,6 @@ func TestMemChunk_IteratorBounds(t *testing.T) {
 		t.Run(
 			fmt.Sprintf("mint:%d,maxt:%d,direction:%s", tt.mint.UnixNano(), tt.maxt.UnixNano(), tt.direction),
 			func(t *testing.T) {
-				t.Parallel()
-
 				tt := tt
 				c := createChunk()