math/big: don't require runtime.(*Frame).Next symbol present

math/big: don't require runtime.(*Frame).Next symbol present

I don't know why the test requires runtime.(*Frame).Next symbol
present in the binary under test. I assume it is just some
sanity check? With CL 268479 runtime.(*Frame).Next can be pruned
by the linker. Replace it with runtime.main which should always
be present.

May fix the longtest builders.

Change-Id: Id3104c058b2786057ff58be41b1d35aeac2f3073
Reviewed-on: https://go-review.googlesource.com/c/go/+/304431
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>