add test case for fragment spread on union types

tests/basics/unionTypesFragment.ts

@@ -0,0 +1,28 @@
+import { readFileSync } from 'fs';
+import * as path from 'path';
+import { graphqlToOpenApi } from '../../index';
+import * as assert from 'assert';
+import * as stringify from 'json-stable-stringify';
+
+describe('unionTypes', function () {
+  it('should produce a valid openapi spec', function () {
+    const schema = readFileSync(
+      path.join(__dirname, 'unionTypesSchema.graphql')
+    ).toString();
+    const inputQueryFilename = path.join(
+      __dirname,
+      'unionTypesQueryFragment.graphql'
+    );
+    const query = readFileSync(inputQueryFilename).toString();
+    // eslint-disable-next-line @typescript-eslint/no-var-requires
+    const expectedOutput = require('./unionTypes.json');
+    const actualOutput = graphqlToOpenApi({
+      schema,
+      query,
+    }).openApiSchema;
+    const normalizedActualOutput = stringify(actualOutput, { space: '  ' });
+    const normalizedExpectedOutput = stringify(expectedOutput, { space: '  ' });
+    assert.ok(!!actualOutput);
+    assert.equal(normalizedActualOutput, normalizedExpectedOutput);
+  });
+});

tests/basics/unionTypesQueryFragment.graphql

@@ -0,0 +1,35 @@
+fragment FurOrHairFragment on FurOrHair {
+  ... on Fur {
+    color
+  }
+  ... on Hair {
+    color
+    texture
+  }
+}
+
+query queryInvolvingUnionTypes($name: String!) {
+  favoritePets(
+    name: $name
+  ) {
+    ... on Cat {
+      adultFur {
+        color
+        texture
+      }
+    }
+    ... on Dog {
+      puppyFur {
+        ...FurOrHairFragment
+      }
+      adultFurs {
+        ... on Fur {
+          texture
+        }
+        ... on Hair {
+          color
+        }
+      }
+    }
+  }
+}