Test: improve PFCOUNT with multiple keys testing.

An user raised a question about a given behavior of PFCOUNT. Added a
test to show the behavior (union) is correct when most of the items are
in common.

tests/unit/hyperloglog.tcl

@@ -136,10 +136,9 @@ start_server {tags {"hll"}} {
         r pfcount hll
     } {5}
 
-    test {PFCOUNT multiple-keys merge returns cardinality of union} {
+    test {PFCOUNT multiple-keys merge returns cardinality of union #1} {
         r del hll1 hll2 hll3
         for {set x 1} {$x < 10000} {incr x} {
-            # Force dense representation of hll2
             r pfadd hll1 "foo-$x"
             r pfadd hll2 "bar-$x"
             r pfadd hll3 "zap-$x"
@@ -151,6 +150,22 @@ start_server {tags {"hll"}} {
         }
     }
 
+    test {PFCOUNT multiple-keys merge returns cardinality of union #2} {
+        r del hll1 hll2 hll3
+        set elements {}
+        for {set x 1} {$x < 10000} {incr x} {
+            for {set j 1} {$j <= 3} {incr j} {
+                set rint [randomInt 20000]
+                r pfadd hll$j $rint
+                lappend elements $rint
+            }
+        }
+        set realcard [llength [lsort -unique $elements]]
+        set card [r pfcount hll1 hll2 hll3]
+        set err [expr {abs($card-$realcard)}]
+        assert {$err < (double($card)/100)*5}
+    }
+
     test {PFDEBUG GETREG returns the HyperLogLog raw registers} {
         r del hll
         r pfadd hll 1 2 3