helper: guard against negative inputs into random stagger

[shoenig]

This PR modifies RandomStagger to protect against negative input
values. If the given interval is negative, the value returned will
be somewhere in the stratosphere. Instead, treat negative inputs
like zero, returning zero.

Before:

=== RUN   TestCluster_RandomStagger
    interface.go:24: 
        cluster_test.go:27: expected 2409629h13m43.082153551s <= 1s
--- FAIL: TestCluster_RandomStagger (0.00s)

[shoenig]

Just now realizing I got these parameters backwards, shoenig/test#33

[tgross]

fwiw, having the parameters in the same "direction" as the operator feels more intuitive to me. ex. must.GreaterEq(t, a, b) reads like it should be checking a > b. Whereas for something like must.Eq there isn't an obvious expectation vs result in the way the parameters are named anyways.

[tgross]

LGTM

[github-actions]

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