test: explicitly turn off skipLibCheck for typings tests (#34798)

skipLibCheck=false is currently the default (in tsc 3.7.4) but it wouldn't be shocking if the default
changed in the future because skipLibCheck=true makes more sense in almost all scenarios. So just to be
defensive and explicit, I'm setting the flag to false even though it's the current default.

PR Close #34798

integration/typings_test_ts36/tsconfig.json

@@ -1,6 +1,7 @@
 {
   "compilerOptions": {
     "strict": true,
+    "skipLibCheck": false,
     "emitDecoratorMetadata": true,
     "experimentalDecorators": true,
     "module": "commonjs",

integration/typings_test_ts37/tsconfig.json

@@ -1,6 +1,7 @@
 {
   "compilerOptions": {
     "strict": true,
+    "skipLibCheck": false,
     "emitDecoratorMetadata": true,
     "experimentalDecorators": true,
     "module": "commonjs",