fix(package.json) watch:tests script to work with Windows

[tja4472]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/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?

yarn run watch:tests script does not work on Windows.

PS C:\VSCode\2019\git-hub-forks\platform> yarn run watch:tests
yarn run v1.13.0
$ chokidar 'modules/**/*.ts' --initial -c 'yarn run test:unit'
Watching "'modules/**/*.ts'", "run", "test:unit'" ..
Terminate batch job (Y/N)? y

Closes #
#1559

What is the new behavior?

PS C:\VSCode\2019\temp\platform-rex> yarn run watch:tests
yarn run v1.13.0
$ chokidar "modules/**/*.ts" --initial --command "yarn run test:unit"
add:modules\entity\index.ts
add:modules\entity\public_api.ts
add:modules\effects\index.ts

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[coveralls]

Coverage remained the same at 89.43% when pulling b072185 on tja4472:issue-1559 into e07ae3d on ngrx:master.

[ngrxbot]

Preview docs changes for b072185 at https://previews.ngrx.io/pr1560-b072185/

[timdeschryver]

LGTM 👍