[schema registry] Facilitate live test runs on arbitrary endpoint (Azure#13030)

* Parameterize schema registry group. It was hard-coded, which
  bumped up against 1 group per resource limit in cheapest tier.

* Remove test of mutating schema and re-registering. When the
  schema changes back and forth between runs like that, it causes
  the service to keep bumping the schema version, which
  eventually hits a limit on total number of schemas.

Author: nguerrera

sdk/schemaregistry/schema-registry/karma.conf.js

@@ -57,6 +57,7 @@ module.exports = function(config) {
     envPreprocessor: [
       "TEST_MODE",
       "SCHEMA_REGISTRY_ENDPOINT",
+      "SCHEMA_REGISTRY_GROUP",
       "AZURE_CLIENT_ID",
       "AZURE_CLIENT_SECRET",
       "AZURE_TENANT_ID"