Add test for create secret

cmd/flux/create_secret_git.go

cmd/flux/create_secret_git_test.go

cmd/flux/create_secret_helm.go

cmd/flux/create_secret_helm_test.go

cmd/flux/create_secret_tls.go

cmd/flux/create_secret_tls_test.go

cmd/flux/main.go

cmd/flux/main_test.go

...x/testdata/create_secret/git/git-ssh-secret-password.yaml

cmd/flux/testdata/create_secret/git/git-ssh-secret.yaml

cmd/flux/testdata/create_secret/git/rsa-password.private

cmd/flux/testdata/create_secret/git/rsa.private

cmd/flux/testdata/create_secret/git/secret-git-basic.yaml

cmd/flux/testdata/create_secret/helm/secret-helm.yaml

cmd/flux/testdata/create_secret/tls/secret-tls.yaml

cmd/flux/testdata/create_secret/tls/test-cert.pem

cmd/flux/testdata/create_secret/tls/test-key.pem