add basic REST test for geohash_grid (#37996)

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/280_geohash_grid.yml

@@ -0,0 +1,62 @@
+setup:
+  - do:
+      indices.create:
+        include_type_name: false
+        index: test_1
+        body:
+          settings:
+            number_of_replicas: 0
+          mappings:
+            properties:
+              location:
+                type: geo_point
+
+---
+"Basic test":
+  - do:
+      bulk:
+        refresh: true
+        body:
+          - index:
+              _index: test_1
+              _id:    1
+          - location: "52.374081,4.912350"
+          - index:
+              _index: test_1
+              _id:    2
+          - location: "52.369219,4.901618"
+          - index:
+              _index: test_1
+              _id:    3
+          - location: "52.371667,4.914722"
+          - index:
+              _index: test_1
+              _id:    4
+          - location: "51.222900,4.405200"
+          - index:
+              _index: test_1
+              _id:    5
+          - location: "48.861111,2.336389"
+          - index:
+              _index: test_1
+              _id:    6
+          - location: "48.860000,2.327000"
+
+  - do:
+      search:
+        rest_total_hits_as_int: true
+        body:
+          aggregations:
+            grid:
+              geohash_grid:
+                field: location
+                precision: 3
+
+
+  - match: { hits.total: 6 }
+  - match: { aggregations.grid.buckets.0.key: u17 }
+  - match: { aggregations.grid.buckets.0.doc_count: 3 }
+  - match: { aggregations.grid.buckets.1.key: u09 }
+  - match: { aggregations.grid.buckets.1.doc_count: 2 }
+  - match: { aggregations.grid.buckets.2.key: u15 }
+  - match: { aggregations.grid.buckets.2.doc_count: 1 }