postings: Fix bad test and revert changed logic

A bad test was introduced a couple of commits ago which also made
changes to pass the bad test. Fixed the test and revert the changes.
Signed-off-by: NGoutham Veeramachaneni <cs14btech11014@iith.ac.in>

postings.go

@@ -201,30 +201,7 @@ func (it *mergedPostings) Seek(id uint32) bool {
 	it.aok = it.a.Seek(id)
 	it.bok = it.b.Seek(id)
 
-	if !it.aok && !it.bok {
-		return false
-	}
-
-	if !it.aok {
-		it.cur = it.b.At()
-
-		return true
-	}
-	if !it.bok {
-		it.cur = it.a.At()
-
-		return true
-	}
-
-	acur, bcur := it.a.At(), it.b.At()
-
-	if acur < bcur {
-		it.cur = acur
-	} else {
-		it.cur = bcur
-	}
-
-	return true
+	return it.Next()
 }
 
 func (it *mergedPostings) Err() error {

postings_test.go

@@ -219,9 +219,18 @@ func TestMerge(t *testing.T) {
 
 			require.Equal(t, c.success, p.Seek(c.seek))
 
-			res, err := expandPostings(p)
-			require.NoError(t, err)
-			require.Equal(t, c.res, res)
+			if c.success {
+				// check the current element and then proceed to check the rest.
+				i := 0
+				require.Equal(t, c.res[i], p.At())
+
+				for p.Next() {
+					i++
+					require.Equal(t, int(c.res[i]), int(p.At()))
+				}
+
+				require.Equal(t, len(c.res)-1, i)
+			}
 		}
 
 		return