Move test tsconfig.json files under __tests__ to support editors

The TypeScript language server looks for `tsconfig.json` and doesn't take project references into account when resolving the config for a file.

See microsoft/TypeScript#27372.

Author: martijnwalraven

src/__tests__/tsconfig.json

@@ -0,0 +1,8 @@
+{
+  "extends": "../../../../tsconfig.test.base",
+  "include": ["**/*"],
+  "references": [
+    { "path": "../../tsconfig.json" },
+    { "path": "../../../apollo-server-integration-testsuite" }
+  ]
+}