fix: set correct dist paths for testing packages

[timdeschryver]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https:/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Closes #3248

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

This PR changes the package.json from the testing (store and effects) packages.
Instead of setting the paths manually, the ng-packagr can do it for us - docs

After this change the output files look like this:

Store:

Before:

{
  "name": "@ngrx/store/testing",
  "typings": "ngrx-store-testing.d.ts",
  "main": "../bundles/core-testing.umd.js",
  "module": "../fesm2015/ngrx-store-testing.mjs",
  "es2015": "../fesm2015/testing.js",
  "esm5": "../esm5/testing/testing.js",
  "esm2015": "../esm2015/testing/testing.js",
  "fesm5": "../fesm5/testing.js",
  "fesm2015": "../fesm2015/ngrx-store-testing.mjs",
  "es2020": "../fesm2020/ngrx-store-testing.mjs",
  "esm2020": "../esm2020/testing/ngrx-store-testing.mjs",
  "fesm2020": "../fesm2020/ngrx-store-testing.mjs",
  "sideEffects": false
}

After:

{
  "module": "../fesm2015/ngrx-store-testing.mjs",
  "es2020": "../fesm2020/ngrx-store-testing.mjs",
  "esm2020": "../esm2020/testing/ngrx-store-testing.mjs",
  "fesm2020": "../fesm2020/ngrx-store-testing.mjs",
  "fesm2015": "../fesm2015/ngrx-store-testing.mjs",
  "typings": "ngrx-store-testing.d.ts",
  "sideEffects": false,
  "name": "@ngrx/store/testing"
}

Effects:

Before:

{
  "name": "@ngrx/effects/testing",
  "typings": "ngrx-effects-testing.d.ts",
  "main": "../bundles/core-testing.umd.js",
  "module": "../fesm2015/ngrx-effects-testing.mjs",
  "es2015": "../fesm2015/testing.js",
  "esm5": "../esm5/testing/testing.js",
  "esm2015": "../esm2015/testing/testing.js",
  "fesm5": "../fesm5/testing.js",
  "fesm2015": "../fesm2015/ngrx-effects-testing.mjs",
  "es2020": "../fesm2020/ngrx-effects-testing.mjs",
  "esm2020": "../esm2020/testing/ngrx-effects-testing.mjs",
  "fesm2020": "../fesm2020/ngrx-effects-testing.mjs",
  "sideEffects": false
}

After:

{
  "module": "../fesm2015/ngrx-effects-testing.mjs",
  "es2020": "../fesm2020/ngrx-effects-testing.mjs",
  "esm2020": "../esm2020/testing/ngrx-effects-testing.mjs",
  "fesm2020": "../fesm2020/ngrx-effects-testing.mjs",
  "fesm2015": "../fesm2015/ngrx-effects-testing.mjs",
  "typings": "ngrx-effects-testing.d.ts",
  "sideEffects": false,
  "name": "@ngrx/effects/testing"
}