Generate certificates for peer CLI tests instead of using cert fixtures

For example, this test in internal/peer/common uses certs that were generated and checked in to the testdata folder: https://github.com/hyperledger/fabric/blob/master/internal/peer/common/peerclient_test.go#L177-L178

We should instead use `tlsgen` (here) to generate the certs/keys and write them to a temp folder.

The goal of this task is to have the same test coverage while removing the testdata/certs folder.