Add missing testing dependency

[nsmith5]

From https://sigstore.slack.com/archives/C02K0T1LNPQ/p1645575739690499:

go test -v ./... fails on main for me, but I can't tell why this wasn't caught in CI 🤔

$ go test -v ./...
go: github.com/sigstore/fulcio/pkg/oauthflow: package github.com/magiconair/properties/assert imported from implicitly required module; to add missing requirements, run:
  go get github.com/magiconair/properties/assert@v1.8.5

go mod tidy indeed generates a diff adding this dependency so I'll open a PR to add this, but I'm curious about why CI isn't having a bad time....

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