big: fix ProbablyPrime bug, comments
(changes adopted from alc, agl)
R=agl1, agl
CC=golang-dev
https://golang.org/cl/181137
diff --git a/src/pkg/big/int_test.go b/src/pkg/big/int_test.go
index b2c33fc..c178bab 100644
--- a/src/pkg/big/int_test.go
+++ b/src/pkg/big/int_test.go
@@ -474,8 +474,15 @@
"5",
"7",
"11",
+
+ "13756265695458089029",
+ "13496181268022124907",
+ "10953742525620032441",
+ "17908251027575790097",
+
"98920366548084643601728869055592650835572950932266967461790948584315647051443",
"94560208308847015747498523884063394671606671904944666360068158221458669711639",
+
// http://primes.utm.edu/lists/small/small3.html
"449417999055441493994709297093108513015373787049558499205492347871729927573118262811508386655998299074566974373711472560655026288668094291699357843464363003144674940345912431129144354948751003607115263071543163",
"230975859993204150666423538988557839555560243929065415434980904258310530753006723857139742334640122533598517597674807096648905501653461687601339782814316124971547968912893214002992086353183070342498989426570593",
