build: first pass of de-duplicating tsconfig.json content (#14369)

PR Close #14369

modules/@angular/compiler/tsconfig-testing.json

@@ -1,23 +1,14 @@
 {
+  "extends": "./tsconfig-build",
+
   "compilerOptions": {
-    "baseUrl": ".",
-    "declaration": true,
-    "stripInternal": true,
-    "experimentalDecorators": true,
-    "module": "es2015",
-    "moduleResolution": "node",
-    "outDir": "../../../dist/packages-dist/compiler/",
+    // Test that we rely on decorator downleveling
+    "emitDecoratorMetadata": false,
     "paths": {
       "@angular/core": ["../../../dist/packages-dist/core"],
       "@angular/core/testing": ["../../../dist/packages-dist/core/testing"],
       "@angular/compiler": ["../../../dist/packages-dist/compiler"]
-    },
-    "rootDir": ".",
-    "sourceMap": true,
-    "inlineSources": true,
-    "lib": ["es6", "dom"],
-    "target": "es5",
-    "skipLibCheck": true
+    }
   },
   "files": [
     "testing/index.ts",