Tests don't build on MacOS or Windows

Found this when migrating to GitHub actions

https://github.com/google/go-attestation/pull/239

E.g. setupSimulatedTPM is only defined on Linux but is referenced in other files

```
attest/certification_test.go: sim, tpm := setupSimulatedTPM(t)
attest/certification_test.go: sim, tpm := setupSimulatedTPM(t)
attest/attest_simulated_tpm20_test.go:func setupSimulatedTPM(t *testing.T) (*simulator.Simulator, *TPM) {
attest/attest_simulated_tpm20_test.go: sim, tpm := setupSimulatedTPM(t)
attest/attest_simulated_tpm20_test.go: sim, tpm := setupSimulatedTPM(t)
attest/attest_simulated_tpm20_test.go: sim, tpm := setupSimulatedTPM(t)
attest/attest_simulated_tpm20_test.go: sim, tpm := setupSimulatedTPM(t)
attest/attest_simulated_tpm20_test.go: sim, tpm := setupSimulatedTPM(t)
attest/attest_simulated_tpm20_test.go: sim, tpm := setupSimulatedTPM(t)
attest/attest_simulated_tpm20_test.go: sim, tpm := setupSimulatedTPM(t)
attest/attest_simulated_tpm20_test.go: sim, tpm := setupSimulatedTPM(t)
attest/attest_simulated_tpm20_test.go: sim, tpm := setupSimulatedTPM(t)
attest/application_key_test.go: sim, tpm := setupSimulatedTPM(t)
attest/application_key_test.go: sim, tpm := setupSimulatedTPM(t)
attest/application_key_test.go: sim, tpm := setupSimulatedTPM(t)
```

Running go test ./... on MacOS or Windows should both build and pass, skipping anything that requires Linux dependencies.

Run go test ./... on macOS or Windows and inspect the platform-specific definitions and references in attest/attest_simulated_tpm20_test.go, attest/certification_test.go, and attest/application_key_test.go. The work is done when the suite builds and passes on both platforms while skipping tests that require Linux dependencies.

Written by the indexing model from the issue text.