PT-1554 Make QueryFormater consistent

In some OSes/Perl versions, the 'filtered' row is being printed with or
without decimals.
For example, in Ubuntu 16.04/Perl 5.22 it is 100.00 while in Ubuntu
18.04/Perl 5.26 it is 100.
To make it consistent for tests, add a default formating string for the
filtered row.

t/pt-query-digest/samples/issue_1196-output-5.7.txt

@@ -45,7 +45,7 @@ select t.a, count(*) from t join t t2 using(a) group by 1 order by 2 desc limit
 #       key_len: NULL
 #           ref: NULL
 #          rows: 14
-#      filtered: 100
+#      filtered: 100.00
 #         Extra: Using temporary; Using filesort
 # *************************** 2. row ***************************
 #            id: 1
@@ -58,5 +58,5 @@ select t.a, count(*) from t join t t2 using(a) group by 1 order by 2 desc limit
 #       key_len: NULL
 #           ref: NULL
 #          rows: 14
-#      filtered: 10
+#      filtered: 10.00
 #         Extra: Using where; Using join buffer (Block Nested Loop)

t/pt-query-digest/samples/slow007_explain_1-57.txt

@@ -41,5 +41,5 @@ SELECT fruit FROM trees\G
 #       key_len: 27
 #           ref: NULL
 #          rows: 1
-#      filtered: 100
+#      filtered: 100.00
 #         Extra: Using index

t/pt-query-digest/samples/slow007_explain_2-57.txt

@@ -41,5 +41,5 @@ SELECT fruit FROM trees\G
 #       key_len: 27
 #           ref: NULL
 #          rows: 3
-#      filtered: 100
+#      filtered: 100.00
 #         Extra: Using index