Add test-target-branch stage to drone again (#11799)

**What this PR does / why we need it**:

Adding the stage back after removing it in
https://github.com/grafana/loki/pull/11785 to fix the test failing in
main.

.drone/drone.jsonnet

@@ -610,13 +610,13 @@ local build_image_tag = '0.33.0';
         'cd -',
       ]) { depends_on: ['clone'], when: onPRs },
       make('test', container=false) { depends_on: ['clone-target-branch', 'check-generated-files'] },
-      //run('test-target-branch', commands=['cd ../loki-target-branch && BUILD_IN_CONTAINER=false make test']) { depends_on: ['clone-target-branch'], when: onPRs },
+      run('test-target-branch', commands=['cd ../loki-target-branch && BUILD_IN_CONTAINER=false make test']) { depends_on: ['clone-target-branch'], when: onPRs },
       make('compare-coverage', container=false, args=[
         'old=../loki-target-branch/test_results.txt',
         'new=test_results.txt',
         'packages=ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki',
         '> diff.txt',
-      ]) { depends_on: ['test' /*, 'test-target-branch' */], when: onPRs },
+      ]) { depends_on: ['test', 'test-target-branch'], when: onPRs },
       run('report-coverage', commands=[
         "total_diff=$(sed 's/%//' diff.txt | awk '{sum+=$3;}END{print sum;}')",
         'if [ $total_diff = 0 ]; then exit 0; fi',

.drone/drone.yml

@@ -212,12 +212,23 @@ steps:
   environment: {}
   image: grafana/loki-build-image:0.33.0
   name: test
+- commands:
+  - cd ../loki-target-branch && BUILD_IN_CONTAINER=false make test
+  depends_on:
+  - clone-target-branch
+  environment: {}
+  image: grafana/loki-build-image:0.33.0
+  name: test-target-branch
+  when:
+    event:
+    - pull_request
 - commands:
   - make BUILD_IN_CONTAINER=false compare-coverage old=../loki-target-branch/test_results.txt
     new=test_results.txt packages=ingester,distributor,querier,querier/queryrange,iter,storage,chunkenc,logql,loki
     > diff.txt
   depends_on:
   - test
+  - test-target-branch
   environment: {}
   image: grafana/loki-build-image:0.33.0
   name: compare-coverage
@@ -2102,6 +2113,6 @@ kind: secret
 name: gpg_private_key
 ---
 kind: signature
-hmac: 323c705d2c43805afcd46813fb9777c7d7d51f7d79aa018e7e78f208eb9f6713
+hmac: 457592d17208477ceb480f81dbdb88f7b95a5ad015c88d9d6fed06c2422a52f9
 
 ...