Downsampling: add more tests for downsample action

x-pack/plugin/rollup/qa/rest/src/yamlRestTest/resources/rest-api-spec/test/rollup/10_basic.yml

@@ -146,6 +146,15 @@ setup:
       indices.get:
         index: test
 
+  # Assert rollup index has been force merged
+  - do:
+      indices.segments:
+        index: rollup-test
+
+  - match:   { _shards.total: 1}
+  - match:   { indices.rollup-test.shards.0.0.num_committed_segments: 1}
+  - match:   { indices.rollup-test.shards.0.0.num_search_segments: 1}
+
 ---
 "Rollup non-existing index":
   - skip:
@@ -181,3 +190,67 @@ setup:
           {
             "fixed_interval": "1h"
           }
+
+---
+"Rollup not time_series index":
+  - skip:
+      version: " - 8.4.99"
+      reason: "rollup renamed to downsample in 8.5.0"
+
+  - do:
+      indices.create:
+        index: normal-index
+
+  - do:
+      catch: /Rollup requires setting \[index.mode=time_series\] for index \[normal-index\]/
+      indices.downsample:
+        index: normal-index
+        target_index: rollup-test
+        body:  >
+          {
+            "fixed_interval": "1h"
+          }
+
+
+---
+"Rollup no metric index":
+  - skip:
+      version: " - 8.4.99"
+      reason: "rollup renamed to downsample in 8.5.0"
+
+  - do:
+      indices.create:
+        index: no-metric-index
+        body:
+          settings:
+            number_of_shards: 1
+            number_of_replicas: 0
+            index:
+              mode: time_series
+              routing_path: [metricset]
+              time_series:
+                start_time: 2021-04-28T00:00:00Z
+                end_time: 2021-04-29T00:00:00Z
+          mappings:
+            properties:
+              "@timestamp":
+                type: date
+              metricset:
+                type: keyword
+                time_series_dimension: true
+
+  - do:
+      indices.put_settings:
+        index: no-metric-index
+        body:
+          index.blocks.write: true
+
+  - do:
+      catch: /Index \[no-metric-index\] does not contain any metric fields;/
+      indices.downsample:
+        index: no-metric-index
+        target_index: rollup-test
+        body:  >
+          {
+            "fixed_interval": "1h"
+          }