chore: make toolkit tests not break Lerna links (#914)

If the integ tests are run in the source directroy, the "npm install"
command will go up to find package.json and replace its node_modules
directory, thereby breaking the Lerna symlinks and making subsequent
toolkit executions fail because it can't find '@aws-cdk/cx-api'.

Copy out the app directory to a temporary directory and run the test
there, so that the source tree remains undisturbed.

It's a fixed directory name, seems simpler than generating a temporary
directory on each run and then also having to install a cleanup hook.

Author: rix0rrr

Diff for: packages/aws-cdk/integ-tests/common.bash

@@ -13,7 +13,10 @@ function cleanup() {
 
 function setup() {
   cleanup
-  cd app
+  rm -rf /tmp/cdk-integ-test
+  mkdir -p /tmp/cdk-integ-test
+  cp -R app/* /tmp/cdk-integ-test
+  cd /tmp/cdk-integ-test
 
   npm i --no-save @aws-cdk/cdk @aws-cdk/aws-sns
 }