fix: loki tail websocket endpoint

[sreis]

A user reported that it could no longer use the loki tail endpoint with the latest version. This regression was introduced with #1140
because in microservices mode, /loki/api/v1/tail is exposed by the querier.

PR also adds a test.

[jgehrcke]

PR also adds a test.

The only right thing. :) Thank you!

[sreis]

AWS test hit a known flake #1226.

Confirmation the new test ran:

[2021-08-26T12:35:27Z]   Loki tail API test suite
[2021-08-26T12:35:27Z] 2021-08-26T12:35:27.154Z info: suite setup
[2021-08-26T12:35:27Z] 2021-08-26T12:35:27.154Z info: globalTestSuiteSetupOnce()
[2021-08-26T12:35:27Z] 2021-08-26T12:35:27.155Z info: Connecting to websocket url: wss://loki.system.prs-bk-5633-6db-a.opstrace.io/loki/api/v1/tail?query=%7Bk8s_namespace_name%3D%22cortex%22%2Ck8s_container_name%3D%22ingester%22%7D
[2021-08-26T12:35:27Z] 2021-08-26T12:35:27.157Z info: waiting 30s for a websocket message...
[2021-08-26T12:35:27Z] 2021-08-26T12:35:27.261Z info: got a message over websocket: {"streams":[{"stream":{"k8s_namespace_name":"cortex","k8s_pod_name":"ingester-1","k8s_container_name":"ingester","k8s_host":"ip-192-168-47-86.us-west-2.compute.internal"},"values":
[2021-08-26T12:35:27Z]     ✓ connect and stream cortex ingester pod logs from /loki/api/v1/tail (107ms)