Move test code into a _test.go file

[rosstimothy]

The lack of a _test.go suffix on the dsa test prevents dead code elimination from removing the test file from builds. This results in testing symbols leaking into every downstream binary that depends on pkcs7 either directly or transitively. For an example see the following dependency tree.

goda tree "reach(github.com/sigstore/sigstore-go/pkg/tlog...:all, testing)"
  ├ github.com/sigstore/sigstore-go/pkg/tlog
    ├ github.com/sigstore/rekor/pkg/types/dsse/v0.0.1
      └ github.com/in-toto/in-toto-golang/in_toto
        └ github.com/secure-systems-lab/go-securesystemslib/signerverifier
    ├ github.com/sigstore/rekor/pkg/types/intoto/v0.0.2
      └ github.com/in-toto/in-toto-golang/in_toto ~
    └ github.com/sigstore/sigstore-go/pkg/root
      └ github.com/sigstore/timestamp-authority/pkg/verification
        ├ github.com/digitorus/pkcs7
        └ github.com/digitorus/timestamp
          └ github.com/digitorus/pkcs7 ~

[rosstimothy]

Hello! @vanbroup, I am a first time contributor to this project, I noticed that you have helped review previous PRs. Is this a PR that you would be able to help provide feedback on? Thank you!