test: Convert and move out of place explain tests (sourcenetwork#1878)

## Relevant issue(s)
Resolves sourcenetwork#1547 

## Description
- Move some explain tests that should have been under the explain
directory, they seemed to have been missed converting to the new explain
setup.

tests/integration/explain/default/with_average_join_test.go

@@ -347,3 +347,49 @@ func TestDefaultExplainRequestWithAverageOnMultipleJoinedFieldsWithFilter(t *tes
 
 	explainUtils.ExecuteTestCase(t, test)
 }
+
+// This test asserts that only a single index join is used (not parallelNode) because the
+// _avg reuses the rendered join as they have matching filters (average adds a ne nil filter).
+func TestDefaultExplainRequestOneToManyWithAverageAndChildNeNilFilterSharesJoinField(t *testing.T) {
+	test := testUtils.TestCase{
+
+		Description: "Explain (default) 1-to-M relation request from many side with average filter shared.",
+
+		Actions: []any{
+			explainUtils.SchemaForExplainTests,
+
+			testUtils.ExplainRequest{
+
+				Request: `query @explain {
+					Author {
+						name
+						_avg(books: {field: rating})
+						books(filter: {rating: {_ne: null}}){
+							name
+						}
+					}
+				}`,
+
+				ExpectedPatterns: []dataMap{
+					{
+						"explain": dataMap{
+							"selectTopNode": dataMap{
+								"averageNode": dataMap{
+									"countNode": dataMap{
+										"sumNode": dataMap{
+											"selectNode": dataMap{
+												"typeIndexJoin": normalTypeJoinPattern,
+											},
+										},
+									},
+								},
+							},
+						},
+					},
+				},
+			},
+		},
+	}
+
+	explainUtils.ExecuteTestCase(t, test)
+}

tests/integration/explain/default/with_count_join_test.go

@@ -260,3 +260,94 @@ func TestDefaultExplainRequestWithCountOnOneToManyJoinedFieldWithManySources(t *
 
 	explainUtils.ExecuteTestCase(t, test)
 }
+
+// This test asserts that only a single index join is used (not parallelNode) because the
+// _count reuses the rendered join as they have matching filters.
+func TestDefaultExplainRequestOneToManyWithCountWithFilterAndChildFilterSharesJoinField(t *testing.T) {
+	test := testUtils.TestCase{
+
+		Description: "Explain (default) 1-to-M relation request from many side with count filter shared.",
+
+		Actions: []any{
+			explainUtils.SchemaForExplainTests,
+
+			testUtils.ExplainRequest{
+
+				Request: `query @explain {
+					Author {
+						name
+						_count(books: {filter: {rating: {_ne: null}}})
+						books(filter: {rating: {_ne: null}}){
+							name
+						}
+					}
+				}`,
+
+				ExpectedPatterns: []dataMap{
+					{
+						"explain": dataMap{
+							"selectTopNode": dataMap{
+								"countNode": dataMap{
+									"selectNode": dataMap{
+										"typeIndexJoin": normalTypeJoinPattern,
+									},
+								},
+							},
+						},
+					},
+				},
+			},
+		},
+	}
+
+	explainUtils.ExecuteTestCase(t, test)
+}
+
+// This test asserts that two joins are used (with parallelNode) because _count cannot
+// reuse the rendered join as they dont have matching filters.
+func TestDefaultExplainRequestOneToManyWithCountAndChildFilterDoesNotShareJoinField(t *testing.T) {
+	test := testUtils.TestCase{
+
+		Description: "Explain (default) 1-to-M relation request from many side with count filter not shared.",
+
+		Actions: []any{
+			explainUtils.SchemaForExplainTests,
+
+			testUtils.ExplainRequest{
+
+				Request: `query @explain {
+					Author {
+						name
+						_count(books: {})
+						books(filter: {rating: {_ne: null}}){
+							name
+						}
+					}
+				}`,
+
+				ExpectedPatterns: []dataMap{
+					{
+						"explain": dataMap{
+							"selectTopNode": dataMap{
+								"countNode": dataMap{
+									"selectNode": dataMap{
+										"parallelNode": []dataMap{
+											{
+												"typeIndexJoin": normalTypeJoinPattern,
+											},
+											{
+												"typeIndexJoin": normalTypeJoinPattern,
+											},
+										},
+									},
+								},
+							},
+						},
+					},
+				},
+			},
+		},
+	}
+
+	explainUtils.ExecuteTestCase(t, test)
+}

tests/integration/explain/fixture.go

@@ -27,6 +27,7 @@ var SchemaForExplainTests = testUtils.SchemaUpdate{
 		type Book {
 			name: String
 			author: Author
+			rating: Float
 			pages: Int
 			chapterPages: [Int!]
 		}

tests/integration/query/one_to_many/utils.go

@@ -16,8 +16,6 @@ import (
 	testUtils "github.com/sourcenetwork/defradb/tests/integration"
 )
 
-type dataMap = map[string]any
-
 var bookAuthorGQLSchema = (`
 	type Book {
 		name: String