Allow cloud tests to be run in parallel

Add the `-parallel N` switch to tell the tests to run in N processes:

```
TFE_TOKEN=$TFE_TOKEN TFE_HOSTNAME=$TFE_HOSTNAME TF_ACC=1 go test -v \
  -tags=e2e ./internal/cloud/e2e/... -parallel 4
```

internal/cloud/e2e/backend_apply_before_init_test.go

@@ -14,6 +14,7 @@ import (
 )
 
 func Test_backend_apply_before_init(t *testing.T) {
+	t.Parallel()
 	skipWithoutRemoteTerraformVersion(t)
 
 	cases := map[string]struct {