chore: fix command to run integ tests (#20414)

The instructions in `CONTRIBUTING.md`, telling users to run `test.sh`,
started running the unit tests instead of the integ tests, since `npx`
now `cd`s to the `package.json` directory (instead of running in the
current directory).

Fix by using `npx` to look up the location of `jest`, but running
it in the current shell.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/aws-cdk/test/integ/common/jest-test.bash

@@ -11,5 +11,8 @@ function invokeJest() {
   # This must --runInBand because parallelism is arranged for inside the tests
   # themselves and they must run in the same process in order to coordinate to
   # make sure no 2 tests use the same region at the same time.
-  npx jest --runInBand --verbose "$@"
+  #
+  # Jest is run in a weird way because npx started (NPM 8?) to change directory
+  # into 'package.json' root, which we don't want here.
+  $(npx which jest) --runInBand --verbose "$@"
 }