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/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