Fix flakes due to 'shared dir' in grpcproxy e2e tests

[ptabor]

Multiple proxy instances were using the same directory in grpcproxy e2e tests, leading to test flakes.

In particular it manifested as:

/home/ptab/corp/etcd/bin/etcd-1483016: {"level":"warn","ts":"2021-05-13T22:50:21.219+0200","caller":"rafthttp/peer_status.go:66","msg":"peer became inactive (message send to peer failed)","peer-id":"5ac8aa22f1eb4c8f","error":"failed to read 5ac8aa22f1eb4c8f on stream Message (unexpected EOF)"}
/home/ptab/corp/etcd/bin/etcd-1483016: {"level":"warn","ts":"2021-05-13T22:50:21.220+0200","caller":"rafthttp/stream.go:421","msg":"lost TCP streaming connection with remote peer","stream-reader-type":"stream MsgApp v2","local-member-id":"72e86eb5e356a737","remote-peer-id":"5ac8aa22f1eb4c8f","error":"unexpected EOF"}
    ctl_v3_test.go:223: could not start etcd process cluster (Cannot start: match not found. Set EXPECT_DEBUG for more info Err: read /dev/ptmx: input/output error, last lines:
        {"level":"warn","ts":"2021-05-13T22:50:21.017+0200","caller":"transport/listener.go:249","msg":"automatically generate certificates","certificate-validity-bound-not-after":"2022-05-13T22:50:21.017+0200"}
        {"level":"info","ts":"2021-05-13T22:50:21.191+0200","caller":"transport/listener.go:298","msg":"created cert file","path":"/tmp/TestCtlV3MemberRemoveClientAutoTLS2632588310/001/default.proxy/fixtures/proxy/cert.pem"}
        {"level":"info","ts":"2021-05-13T22:50:21.192+0200","caller":"transport/listener.go:319","msg":"created key file","path":"/tmp/TestCtlV3MemberRemoveClientAutoTLS2632588310/001/default.proxy/fixtures/proxy/key.pem"}
        {"level":"info","ts":"2021-05-13T22:50:21.192+0200","caller":"etcdmain/grpc_proxy.go:201","msg":"gRPC proxy server TLS","tls-info":"cert = /tmp/TestCtlV3MemberRemoveClientAutoTLS2632588310/001/default.proxy/fixtures/proxy/cert.pem, key = /tmp/TestCtlV3MemberRemoveClientAutoTLS2632588310/001/default.proxy/fixtures/proxy/key.pem, client-cert=/tmp/TestCtlV3MemberRemoveClientAutoTLS2632588310/001/default.proxy/fixtures/proxy/cert.pem, client-key=/tmp/TestCtlV3MemberRemoveClientAutoTLS2632588310/001/default.proxy/fixtures/proxy/key.pem, trusted-ca = , client-cert-auth = false, crl-file = "}
        {"level":"fatal","ts":"2021-05-13T22:50:21.202+0200","caller":"etcdmain/grpc_proxy.go:381","msg":"failed to create TLS listener","error":"tls: private key does not match public key","stacktrace":"go.etcd.io/etcd/server/v3/etcdmain.mustListenCMux\n\t/home/ptab/corp/etcd/server/etcdmain/grpc_proxy.go:381\ngo.etcd.io/etcd/server/v3/etcdmain.startGRPCProxy\n\t/home/ptab/corp/etcd/server/etcdmain/grpc_proxy.go:203\ngithub.jparrowsec.cn/spf13/cobra.(*Command).execute\n\t/home/ptab/private/golang/pkg/mod/github.com/spf13/[email protected]/command.go:854\ngithub.jparrowsec.cn/spf13/cobra.(*Command).ExecuteC\n\t/home/ptab/private/golang/pkg/mod/github.com/spf13/[email protected]/command.go:958\ngithub.jparrowsec.cn/spf13/cobra.(*Command).Execute\n\t/home/ptab/private/golang/pkg/mod/github.com/spf13/[email protected]/command.go:895\ngo.etcd.io/etcd/server/v3/etcdmain.Main\n\t/home/ptab/corp/etcd/server/etcdmain/main.go:32\nmain.main\n\t/home/ptab/corp/etcd/server/main.go:32\nruntime.main\n\t/usr/lib/google-golang/src/runtime/proc.go:225"}
        )
--- FAIL: TestCtlV3MemberRemoveClientAutoTLS (2.93s)