math/rand: restore Go 1.2 value stream for Float32, Float64

CL 22730043 fixed a bug in these functions: they could
return 1.0 despite documentation saying otherwise.
But the fix changed the values returned in the non-buggy case too,
which might invalidate programs depending on a particular
stream when using rand.Seed(0) or when passing their own
Source to rand.New.

The example test says:
        // These tests serve as an example but also make sure we don't change
        // the output of the random number generator when given a fixed seed.
so I think there is some justification for thinking we have
promised not to change the values. In any case, there's no point in
changing the values gratuitously: we can easily fix this bug without
changing the values, and so we should.

That CL just changed the test values too, which defeats the
stated purpose, but it was just a comment.
Add an explicit regression test, which might be
a clearer signal next time that we don't want to change
the values.

Fixes #6721. (again)
Fixes #8013.

LGTM=r
R=iant, r
CC=golang-codereviews
https://golang.org/cl/95460049

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