math/rand: use t.Helper in tests
math/rand: use t.Helper in tests

Change-Id: Iece39e6412c0f6c63f563eed1621b8cca02de835
Reviewed-on: https://go-review.googlesource.com/51890
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Avelino <8efd86fb78a56a5145ed7739dcb00c78581c5375@avelino.xxx>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

polyverse-security/go,codestation/go,akutz/go,Cofyc/go,josharian/go,golang/go,golang/go,akutz/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,akutz/go,cloudflare/go,polyverse-security/go,Cofyc/go,josharian/go,golang/go,polyverse-security/go,polyverse-security/go,akutz/go,Cofyc/go,nawawi/go,codestation/go,nawawi/go,polyverse-security/go,codestation/go,golang/go,CAFxX/go,golang/go,codestation/go,nawawi/go,polyverse-security/go,Cofyc/go,cloudflare/go,Cofyc/go,nawawi/go,akutz/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,josharian/go,josharian/go,codestation/go,golang/go,nawawi/go,codestation/go,akutz/go,Cofyc/go,nawawi/go,CAFxX/go,akutz/go,codestation/go,josharian/go,CAFxX/go,polyverse-security/go,josharian/go,golang/go,Cofyc/go,CAFxX/go,polyverse-security/go,josharian/go,christopher-henderson/Go,akutz/go,codestation/go,josharian/go,cloudflare/go,CAFxX/go,Cofyc/go,cloudflare/go,cloudflare/go,cloudflare/go